A verb mood indicates the attitude of the speaker toward the action or state of being of the verb. The main moods in English are the indicative, imperative, and subjunctive.
- Indicative mood expresses a statement of fact (B).
- Imperative mood is used to give commands (C).
- Subjunctive mood can express wishes, hypotheticals, or conditions contrary to fact (D).
So, the best option that explains the function of verb mood overall is:
A. It is how a verb is used to ask a question.
However, if you are seeking to summarize moods more broadly, none of the options comprehensively cover all moods. But "B" also accurately describes a specific use of mood. If the focus is solely on the question, then "A" is appropriate.