"No one knows" makes complete sense and can stand on its own so thats the main clause while "whom Allen hired to paint his house" is the depndent clause as it is missing an important aspect.
Out of all given options, “whom Allen hired to paint his house” identifies as the subordinate clause in the given sentence.
Answer: Option D
<u>Explanation:
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A subordinate clause is a phrase which cannot stand independently, thus being dependent on a sentence. If added it can give more meaning to the sentence. By adding a subordinate clause, it gives more meaning to the sentence.
In this case, 'whom Allen hired to paint his house' is a subordinate clause as this phrase cannot stand independently if made to do so. A subordinate clauses usually initiates with a conjunction. Since it cannot stand independently, subordinate clause is also referred as dependent clause.
