Which one of the following shows a unique relationship with an insect that has coevolved with it and that is the only insect that can pollinate this tree?

The correct answer is A, Fig. Each fig species is pollinated only by its own tiny fig wasp, and the two have evolved together over millions of years. A fig 'fruit' is really a closed bulb, the syconium, with hundreds of small flowers inside. A female wasp squeezes in through a tiny opening, pollinates the flowers and lays her eggs in some of them. The young wasps grow inside, mate, and the new females fly out carrying pollen to another fig. Without the wasp the fig cannot set seed, and without the fig the wasp cannot breed; this is obligate mutualism. Figs such as the banyan and peepal are keystone species, because their fruit feeds many birds and mammals through the year. Option B is wrong because mahua flowers are pollinated mainly by bats and some birds. Option C is wrong because sandalwood, a root hemiparasite, is visited by many kinds of insects, not one. Option D is wrong because silk cotton (semal) is pollinated mainly by birds. Exam tip: fig and fig wasp are the classic example of co-evolution and obligate mutualism.