100ml of gaseous hydrocarbon consumes 300ml of oxygen for the complete combustion. The hydrocarbon is

A. C2H4

B. C2H2

C. C3H8

D. C2H6

Let's look at the balanced equation for C2H4.
C2H4  + 3O2 -->  2CO2 + 2H2O
So 1 mole of this hydrocarbon reacts with 3 moles of oxygen and since  1 mole of one gas occupies the same volume as another gas, the volumes  will be in the ratio 1:3.  So this is the required hydrocarbon.

Hence option A is correct