The correct answer is: "c. The footsteps of my friend are heard by me when I am alone."
In the given sentence, the subject is "footsteps," which is plural. Therefore, the verb used should also be plural, and the correct form of the verb "to be" is "are." The sentence is in the present tense and describes a recurring event, so the present simple tense is used in the sentence. The correct sentence is "The footsteps of my friend are heard by me when I am alone."