Explanation:
The appropriate form of the verb to fill in the blank is "rains."

Subject-Verb Agreement:
In the sentence, the subject is "it" and the verb is "rains." The subject "it" is a pronoun that represents the weather or the conditions in a particular place.

Present Simple Tense:
The verb "rains" is in the present simple tense. This tense is used to talk about general truths, habits, or things that happen regularly.

Usage:
In this sentence, the word "here" implies that the speaker is referring to the current location. Hence, it is stating that it rains regularly all year round in this place.

Example:
To further understand, let's look at an example:
- "It rains all year round here, so make sure to always carry an umbrella."

In this example, the speaker is informing the listener that it is a common occurrence for rain to happen throughout the year in their current location.

Other Options:
- Option A: "Rain" is incorrect because it is not in the correct form. The verb should agree with the subject "it" and be in the present simple tense.
- Option C: "Is raining" is incorrect because it is in the present continuous tense, which implies that the rain is happening at the time of speaking. However, the sentence is referring to a general occurrence throughout the year.
- Option D: "Was raining" is incorrect because it is in the past continuous tense. The sentence is not referring to a past event but rather a present or ongoing situation.