Am attempting to answer your queries: Hope this helps...
1. It was me whom you saw on train yesterday
'A' is incorrect. The sentence shoulkd read "It was I whom ...". An "It was" construction is always followed by the pronoun in its nominative case and not in the objective case.
2) Anybody/ Everybody is plural or not? My teacher once said they are plural but another teacher said they are just singular. ????
Anybody, somebody, everybody and nobody, all are singular.
3)Whom would you rather __________ with you to Hong Kong?
OK, I was confused about this one, but I think the decisive word is "would". "would" is based not in reality but in probability. eg. "I would have done it differently". If you choose "have gone", that action has already taken place. You need to choose something which indicates what may have happened, not what had happened. hence I supposed it should be 'have go' and not 'have gone'.
4) What's the difference between " Explicate- Explain" "explicable and explainable"?
Both are similar in meaning, except that "explicate" means a details explanation. So I suppose explicate = 'explain in detail'
Could anybody please explain to me?