For a compound to be aromatic, it must satisfy Hückel's rule ($4n + 2 \, \pi$ electrons, where $n$ is an integer), and it must be cyclic and planar with a conjugated system.
Compound A is non-aromatic.
Compound B is aromatic as it has a conjugated planar ring with a total of 6 $\pi$ electrons.
Compound C is non-aromatic due to lack of conjugation.
Compound D is aromatic with a conjugated planar ring and 6 $\pi$ electrons.