I looked this question up and, in one of the sites where I found it, each number had two answer choices. Just in case, I will use them here.
1.
A. She may / might / could be working too hard.
B. She must be working too hard.
2.
A. He must not earn very much.
B. He may not / might not earn very much.
Answer:
The answers are:
1. A. She may / might / could be working too hard.
2. B. He may not / might not earn very much.
Explanation:
The modal verbs "may", "might" and "could" are used to express possibility. Therefore, they do not convey much certainty, that is, the speaker is not 100% sure of what he / she is saying, but he / she knows there is a possibility of being right.
That is why those verbs are the best ones to complete the sentences above. In both cases, the speaker is supposing, assuming something. In the first sentence, the speaker believes it is possible that Deborah is tired because she is working too hard. In the second one, the speaker assumes "he" is likely to not earn much because he never goes on holiday.
Note: "Could" is also used to express ability in some cases.
