The correct answer is: A. If Jack was here, he could help us.

Explanation:

The original sentence "Jack isn’t here. He can’t help us." implies that because Jack isn't here, he is unable to help.
A. If Jack was here, he could help us. correctly represents the idea in a conditional form, suggesting that if Jack were present, he would be able to help.
B. If Jack is here, he can help us. is incorrect because it assumes Jack is here in the present, while the original sentence indicates that he is not.
C. If Jack isn’t here, he can’t help us. is a direct restatement of the original sentence and does not fit the intended conditional structure.
D. If Jack weren’t here, he couldn’t help us. is incorrect because it uses the past perfect tense ("weren’t here"), which is not appropriate for this context. The correct form would use "was" in the past subjunctive.
Thus, the most appropriate rephrasing is A.

A. If Jack was here, he could help us.

vì

Đây là câu điều kiện loại 2, dùng để diễn tả tình huống giả định không có thật ở hiện tại
Cấu trúc câu điều kiện loại 2
If + S + past simple, S + could/would + V-infinitive.