Answer: B: mightExplanation:- The sentence is about a possibility or uncertainty, which is best expressed using the modal verb "might."- "Mustn't" implies a prohibition or something that is not allowed.- "Have to" indicates an obligation or necessity.- "Wouldn't" is used to express a refusal or a hypothetical situation in the past.

Modal Verb Explanation:
Modal verbs are used to express various degrees of possibility, necessity, ability, and permission. They modify the main verb in a sentence and help convey the speaker's attitude or opinion regarding the action. In this case, the modal verb is used to express the possibility of going to the concert depending on the weather condition.

Explanation of the correct answer:
The correct modal verb to fill in the blank in this sentence is 'might.'

Explanation:
- 'Mustn't' is used to express prohibition or strong advice. It indicates that going to the concert is not allowed or strongly discouraged. However, the sentence suggests that the speaker is uncertain about going to the concert if the rain stops, so 'mustn't' does not fit the context.
- 'Might' is used to express possibility or uncertainty. It indicates that there is a chance of going to the concert if the rain stops. Since the speaker is unsure and doesn't have definite information about the weather, 'might' is the appropriate modal verb in this sentence.
- 'Have to' is used to express obligation or necessity. It indicates that going to the concert is required or mandatory. However, the sentence does not imply any obligation or necessity, but rather the possibility of going depending on the weather condition.
- 'Wouldn't' is used to express a hypothetical situation or a conditional action. It indicates that the speaker would not go to the concert if the rain stops. However, the sentence suggests that the speaker is uncertain and open to the possibility of going, so 'wouldn't' does not fit the context.

Hence, the correct modal verb to fill in the blank is 'might.'