'Hoist with one's own petard' (from Shakespeare's Hamlet) means to be harmed by one's own plan to harm another, making option A correct. A petard was a medieval explosive device used to breach gates. If the engineer was 'hoist' (lifted up/blown up) by their own petard, they were killed by their own bomb. 'Becoming friends with a sailor' and 'using wrong units' are nonsensical distractors unrelated to the phrase's meaning or origin.