Is the word a a preposition?

Answer 1

It can be, when used to mean "in" or "per" or "in every."

(e.g. The mail comes once a day, once per day).

In English A can be--

• a noun: I got an A in basketweaving.
• an indefinite article: He ate a worm.
• a verb (contraction of "have") I might a eaten a couple worms, too.
• a preposition: Twice a week I read my English text.

Answer 2

No, the word "a" is not a preposition. It is an indefinite article used before nouns to indicate a single item or individual.