The correct answer is: B. rains
The sentence “It rains all year round here” is in the present tense. This means that it is a general statement about the weather in a particular place. The verb “rains” is the simple present tense form of the verb “to rain.” It is used to describe weather conditions that are typical of a particular place.
The other options are incorrect because they are not in the present tense. Option A, “rain,” is the infinitive form of the verb. Option C, “is raining,” is the present progressive tense form of the verb. Option D, “was raining,” is the past tense form of the verb.
In conclusion, the correct answer to the question “Fill in the blank with the appropriate form of the verb. It _____ all year round here.” is: B. rains.