Which statement is​ correct?

A.
The article an often comes before a noun that is​ modified, so an broken piece.
B.
The article an often comes before plural​ nouns, so an alligators.
C.
The article an often comes before nouns that begin with​ consonants, so an tree.
D.
The article an often comes before a noun that is not​ specific, so an animal.