Is the group of words in bold a phrase or a clause?

Question

The earth is not a perfect sphere but rather an oblate sphere because it is flattened at the poles and enlarged at the equator.

Answer 1

The group of words in bold, "because it is flattened at the poles and enlarged at the equator," is a clause. Specifically, it is a dependent (or subordinate) clause because it cannot stand alone as a complete sentence and provides a reason for the statement preceding it.