The correct option is C before
A conjunction's main function is to join or combine two clauses as a statement. In the given sentence, the clauses are identified as "Read the instructions" and "you assemble the parts", where the latter is the subordinate/dependent clause because it depends on the former, the main/independent clause, for its full meaning. They both sound grammatically complete, though the latter isn't complete in meaning. The word that's been left behind is 'before', as it combines the clauses and is not a part of them. 'Before' is a subordinating conjunction and is used for time-related sentences, like the given one, where the events "read" and "assemble" occur one after the other. Hence, option D is correct. 'You' is a second person singular pronoun, it addresses the listener, with whom the speaker is having a conversation. It does not combine the clauses, the clause before and after 'you' sound incomplete, hence it's not a conjunction and option A is incorrect. 'The' is an article that modifies the noun "instructions", it does not combine the sentences, hence option B is incorrect. 'Read' is a verb that describes the action of reading, 'comprehending words in written form'. It does not link the clauses together, hence option C is incorrect.