Possibility Modal:
The modal verb that expresses possibility is "can."

Explanation:
- Modal verbs are a type of auxiliary verb that express various meanings such as possibility, necessity, permission, ability, and more.
- Among the given options, "can" is the modal verb that specifically expresses possibility.
- Let's understand the meaning and usage of each option to further clarify why "can" is the correct answer:

a) Can:
- "Can" is used to indicate that something is possible or someone has the ability to do something.
- It is commonly used to talk about general possibilities or abilities.
- Example: "I can swim." (This means that the person has the ability to swim.)

b) Could:
- "Could" is the past tense form of "can" and is also used to express possibility.
- It is used to talk about hypothetical or less certain possibilities.
- Example: "He could be at the library." (This means that there is a possibility that he is at the library.)

c) Must:
- "Must" is used to express necessity, obligation, or strong probability, but not possibility.
- It indicates that something is highly likely or required to happen.
- Example: "You must study for the exam." (This means that studying for the exam is necessary.)

d) Will:
- "Will" is used to express future actions, intentions, or predictions, but not possibility.
- It indicates that something is going to happen in the future.
- Example: "She will go to the party tonight." (This means that she has made the decision to go to the party.)

Conclusion:
Among the given options, "can" is the modal verb that expresses possibility. It is used to indicate that something is possible or someone has the ability to do something. Other options like "could," "must," and "will" express different meanings such as past possibility, necessity, and future actions respectively.