According to Mackie, if objective moral properties existed they would be queer because...

C. We could never verify whether or not they obtained.

Explanation: According to Mackie, for a claim to be objective, It has to be something that's known and not inferred or guessed, it has to be either categorically true or false, Its truth is independent irrespective of what we want it to be or choose and it must be about something that is part of the 'fabric of the world' i.e be in existence and tangible.

Going by these statements, moral objectivity is not possible as there are no concrete evidence to measure or prove them by. Our morals mostly come from what's acceptable in social and cultural settings. For example, the statement "stealing is wrong" is objectively impossible as the act of stealing is wrong according to society, not that there's a way to prove how we have this knowledge.