0.34 g of hydrocarbon when heated with methyl magnesium bromide gives 112 ml of CH4  at STP. Possible structure of hydrocarbon is

112ml of CH4 at STP is obtained from 0.34g 22400ml of CH4 at STP is obtained from 0.34112×22400=68g  mol−1 So alkyne is C5H8=5×12+8=68g  mol−1 . So correct ans is (a)