Answer:
d- However, if I can improve my writing by studying grammar, the effort will be well worth it!
Explanation:
Auxiliary verbs are helping verbs that add meaning to the main verb. They help express the modality of the verb.
Among the given sentences, the sentence that uses modal auxiliary verb is sentence d. In this sentence, the modal auxiliary verbs are "can" and "will", which express ability.
Thus, the correct answer is option d.
