Properties of subspace:
1.The zero vector is in the subspace
2.Subset is closed under addition and scalar multiplication
Option A,B,D satisfies these properties but option c is not closed under addition , for example
vector1=[1 0 1]^t (where b=0) and vector2=[ 0 1 0 ]^t (where a=0).Their sum is [1 1 1 ]^t which does n't satisfy the condition a=0 or b=0 or both.