You must verify that the number of atoms of each type is equal on both sides of the chemical equation: same number of C, same number of H and same number of O on both sides.
A. C4H6 + 5.5O2 ---> 4CO2 + 3H2O
element reactant side product side
C 4 4
H 6 3*2 = 6
O 5.5 * 2 = 11 4*2 + 3 = 11
Then, this equation is balanced.
Do the same with the other equations if you want to verify that they are not balanced.
Answer: option A.
