Which sentence is correct? Or none is correct?

Yes, 'A' is the correct sentence. The rest of the sentences are incomplete and the meaning of those other sentences can be misinterpreted. Sentence 'B' is not incorrect as it may well be used colloquially in speech, but rarely in a written sentence. Sentence D leads the reader to wonder which last week they are referring to. It could be the last week of a school term, or the last week of Summer, or the week before the current one. It is not definitive enough. Sentence 'D' is similar to sentence 'C', and does not fully define which week the writer is talking about.

A & C look like the same question. B & D look like the same question.

I would say A/C is more colloquially correct, but B/D would still work in casual conversation.

C and D are definitely wrong. A and C are both "almost" correct.

Even better would be: "He had been studying hard for the previous week".

'A' is correct

All of them are good enough English, but I think B. sounds the best myself.

Bouth