The modal verb "must" is used to indicate necessity or obligation.

Necessity and Obligation

Modal verbs are a category of verbs that are used to express various meanings in a sentence. One of the functions of modal verbs is to express necessity or obligation. These modal verbs help to convey the idea that something is required or mandatory.

Explanation:

The modal verb that is commonly used to express necessity or obligation is 'must'. Let's understand why 'must' is the correct answer:

1. Must: 'Must' is used to indicate a strong necessity or obligation. It suggests that there is no alternative or choice in the matter. For example:
- "You must complete your homework before going out to play."
- "I must finish this project by tomorrow."

2. Could: 'Could' is used to express possibility or ability, but it does not convey a strong sense of necessity or obligation. It suggests that something is optional or not mandatory. For example:
- "You could join us for dinner if you want to."
- "Could you please pass me the salt?"

3. Might: 'Might' is used to express possibility or uncertainty, but it does not indicate a strong necessity or obligation. It suggests that something is not certain or definitive. For example:
- "I might go to the party tonight if I finish my work."
- "She might be busy, so I'll call her later."

4. Shall: 'Shall' is used to express future actions or to make suggestions, but it does not specifically convey necessity or obligation. It is more commonly used in formal or legal contexts. For example:
- "We shall meet at the park tomorrow."
- "Shall we go for a walk?"

Therefore, the correct modal verb that is used to express necessity or obligation is 'must'. It is important to understand the specific meanings and usages of different modal verbs in order to convey the intended message accurately.