Is the an adjective

Answer 1

Technically it is called an "article" - it establishes "definiteness" of a noun.

It indicates a specific individual, example or set within a group of things or concepts, for example the red truck versus just any old red truck, which is a red truck.

Answer 2

"The" is a definite article, a determiner. Some grammar books include determiners as types of adjectives.

To be safe, you can simply classify it as a definite article.

Answer 3

Adjective