B is incorrect because the C with the triple bond is shown also single bonded to two hydrogens and another C atom: That would give us a C with 12 bonds, which violates the octet rule.
C is incorrect because you have an sp3 carbon atom that has only two hydrogens, and so is short two bonds. This would be correct if either there were another H bonded to the carbon atom or if it formed a double bond with an adjacent carbon atom (an H would have to be removed from that adjacent carbon, however).
D is incorrect for the reason mentioned above: You have a C that is double-bonded to one C, and single-bonded to another C and two H atoms. This adds up to 10 bonds, violating the octet rule. This would be a correct structure if a hydrogen atom were removed from the middle carbon atom.
