What part of speech is 1999 in the sentence she was born 1999?

In the sentence "She was born 1999", "1999" functions as an adjective.

Here's why:

* Adjectives modify nouns. In this case, "1999" modifies the noun "born", specifying the year of her birth.

* Prepositions often introduce phrases that act as adjectives. The preposition "in" is implied before "1999" (She was born *in* 1999).

* Numbers can sometimes function as adjectives. This is particularly true when they directly describe a noun, as in this case.

While "1999" is technically a numeral, its function in this sentence makes it an adjective.