Are the underlined words in the sentence an independent clause, a subordinate clause, or neither? since early this summer, my friends and i have been riding our bikes every evening.

a. subordinate clause

b. neither

c. independent clause

I'm assuming the first four words are underlined in your text. If this is indeed the case then it is a subordinate clause as it cannot stand alone to make its own sentence and is started with since a subordinating conjunction. (More specifically it is an adjective clause as it describes when the bikes were ridden.)