A noun phrase is a phrase which has a noun or a pronoun as the nucleus and, gramatically, functions as a noun.
Considering this, we can conclude that the correct answer is C: "a relatively short time period" because the nucleus of that phrase is the noun "period".
The rest are not possible options because both A and B are independent clauses which do not complete the sentence but add new independent information, and D is not a noun phrase.
