What part of speech is the italicized word? (rainstorm)

the campers were drenched by the rainstorm.

noun
verb
pronoun
adjective
adverb

The italicized word is a noun.

Explanation:

 A noun is that word that can represent a thing, a person, an animal, a place, a quality, or an idea. Most of the time it is simply a single word.

Nouns can be common or proper, where common nouns refer to people, places or things but in a general way. While a proper noun refers to something particular: Donald Trump, Chile, etc.

Nouns can be countable and uncountable.

As the name implies, countable nouns are those that can be counted: a house, a car, three tables.

While uncountable nouns are those that cannot be counted: water, snow, rain.

On the other hand we have the gerunds, which are those nouns that end in –ing. For example: I love cooking. (cooking is a gerund).

We also have the attributive nouns, which are those that modify another noun, for example: book store.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.