Which was the last Mauryan ruler, killed by his commander Pushyamitra Shunga?

The correct answer is C, Brihadratha. Brihadratha was the last of the Mauryas, and around 185 BCE his own commander-in-chief Pushyamitra Shunga killed him and took the throne, founding the Shunga dynasty with its centres at Pataliputra and Vidisha. The Shungas are remembered for the Sanskrit revival, for Patanjali and for the stone railing and gateways added at Sanchi and Bharhut.

Options A, B and D are all later Mauryas but not the last. Dasharatha, a grandson of Ashoka, is known from the Nagarjuni hill cave inscriptions. Samprati appears in Jaina tradition as a patron of that faith. Shalishuka is mentioned in the Gargi Samhita as an unrighteous ruler. The order of the weak successors is rarely asked, but the name of the last ruler and the date of the Shunga takeover are standard, so remember Brihadratha with 185 BCE.