Pick out the noun phrase from the given sentence:
Both the thieves have fled with the jewels.

• A phrase refers to a word or group of words containing a noun and performing the function of a subject matter, object, or object in an exceeding sentence.
  Let us take a glance at the given options.
• Option ‘B’, thieves, is wrong because it could be a noun and not a phrase.
• Option ‘C’, have fled, is wrong because the word ‘have’ could be a verb and hence it's a predicate.
• Option ‘D’, with the jewels, is wrong because it contains a preposition, ‘with’ and is hence a phrase.
  So, the proper answer is option ‘A’, both the thieves, because it contains a noun ‘thieves’ and a quantifier ‘both’ that's accustomed modify the given noun.

Hence, option A is the correct answer.