Answer:
d). might not be able to help
Step-by-step explanation:
Modal verbs are characterized as auxiliary verbs that aim to reflect the mood of the sentence. The different modal verbs show different moods like seeking or granting permission, advice, showing the probability of an event, ability, duty, etc.
In the given sentence, the correct modal verb to be used would be 'might' in order to neglect the 'possibility of him not helping her' irrespective of his wish whether he wants to do it or not. 'Might' is the modal auxiliary that is used to show the least possible actions or likelihood in past. Since the sentence is framed in past, it stands for 'least possibility' which is further being refuted by use of 'not.'