The last Mauryan ruler to be killed by Pushyamitra Shunga was:

Brihadratha

The correct answer is Option 2 -Brihadratha

The Shunga Dynasty (c. 185 BCE - 73 BCE) rose to power in northern India after the decline of the Maurya Empire. Its founder, Pushyamitra Shunga, was a Brahmin general who assassinated the last Mauryan ruler, Brihadratha, and seized the throne.