Sincerity is a noun.
A big clue is the article "the" before it.
Remember, that nouns name persons, places, things, and ideas.
What is the part of speech for the word sincerity in this sentence. The sincerity of his answer convinced me.
2 answers
The articles, "the, a, an" are like road signs. The always announce that a noun is ahead. Sometimes there are one or two adjectives between them, but you always know when you see one of those road signs that a noun is coming.