They carried their instruments to the stage.
To figure out which word is the subject pronoun in this sentence, let us first look at exactly what the "subject" of a sentence is. The subject of a sentence is the person or thing doing the action. In this sentence, the action (verb) is "carried"; those that did the carrying are the subject - "they". The object of the sentence is being acted upon by the subject (in this case, the instruments - they are being carried by the subject "they"). They is a pronoun, thus it is a subject pronoun in this sentence.
The answer is option c. they.