Question

is the group of words in bold a phrase or a clause?
the solid portion of a comet comprises leftover debris—ice, dust, rock—that originated in the outer regions of the solar system.

Explanation:

A phrase is a group of words without a subject - verb combination, while a clause has a subject and a verb. The bolded group "The solid portion of a comet" has no verb, so it is a phrase.

Answer:

Phrase