The two supermassive black holes at the centre of OJ 287 orbit each other once in how many years?

The correct answer is the Aryabhatta Research Institute of Observational Sciences, Nainital. The multi-institutional campaign on the blazar OJ 287 was coordinated by scientists of this institute, which is an autonomous body of the Department of Science and Technology, and the study was led by Dr Alok C. Gupta of the same institute. The team gathered 106 scientists from 18 countries and 48 institutions and universities, who observed the object with about two dozen telescopes between 2015 and 2025. Options A, C and D name well-known Indian centres for astronomy and space science, but none of them is credited with coordinating this particular study; they are offered because candidates often confuse the national astronomy institutes with one another. The safest way to remember the fact is to link the Nainital institute, its parent department and the name of the lead scientist together, since any of the three can be asked.

The correct answer is 3.89 billion solar masses. Using eight optical spectra recorded at Steward Observatory in the United States during low-brightness phases of the object in October and November 2017, the team estimated the central black hole to weigh no less than that, reading the figure from the width of the oxygen emission line. Option A is wrong because it shifts the same digits down by a factor of a thousand; supermassive black holes can range from millions to billions of solar masses, so a candidate who misses the unit will fall for it. Option C is wrong because eighteen is the number of countries whose scientists joined the study, not a mass. Option D is wrong for the same kind of reason, since one hundred and six is the number of scientists in the team. The lesson is to keep the three large numbers of this study separate: the team size, the number of countries and the mass estimate.

The correct answer is fifth. India has emerged as the fifth nation in the world to design and build an indigenous 4G stack of its own, a milestone the Ministry of Communications treats as proof of growing self-reliance in developing and deploying advanced telecom technology. Options A, B and D give the wrong rank and are the kind of near misses a paper setter uses when a milestone is stated as an ordinal. Along with this achievement, the figures to remember for the country's network are 1.01 billion broadband subscribers and over one billion internet subscribers, 5G services reaching 99.9 per cent of districts and 85 per cent of the population, and 4G connectivity in 98.7 per cent of villages. These numbers were cited while the arrangements for the tenth India Mobile Congress at Yashobhoomi were being reviewed, so the ordinal and the coverage figures are often asked together.

The correct answer is Yashobhoomi, Dwarka, New Delhi. The Union Minister of Communications visited this convention centre to review the final arrangements before the opening of the tenth India Mobile Congress, and the exhibition halls, sessions and investor meetings of the event are housed there. Options A, C and D name other well known venues in the national capital that host conferences and exhibitions, but none of them is the site of this edition of the congress. The distinction matters because questions on summits and expositions almost always pair the edition with its venue and its theme. For this event the three go together as follows: the tenth edition, the theme 'Scale Without Boundaries', and Yashobhoomi in Dwarka as the venue, with the Prime Minister inaugurating the congress on 8 October 2026. Only option B is therefore correct.

The correct answer is Scale Without Boundaries. This is the theme chosen for the tenth India Mobile Congress, which is being held at Yashobhoomi in Dwarka, New Delhi, and which the Prime Minister inaugurates on 8 October 2026. The theme points to the ambition of the edition, which brings together more than 500 exhibitors and partners, over 600 startups and over 900 thought leaders on one platform. Option A, Digital India for All, names a different government programme and is not the theme of this event. Option C, Connecting the Next Billion, resembles the title of the Ministerial Programme at the event, which is 'Investing in the Next Billion Digital Connections', but that is one session and not the theme of the congress itself. Option D, Sovereign Telecom Future, echoes the CEO Roundtable titled 'The Sovereign Inflection and the Future of Telecom', again a single session rather than the theme. Only option B names the theme of the whole edition.