The correct answer is C: By increasing the number of turns and/or the area enclosed by the loop. The output voltage of a multiple-turn receiving loop antenna can be increased by increasing the number of turns and/or the area enclosed by the loop. More turns and larger area increase the voltage induced by the magnetic field. The voltage induced in a loop antenna is proportional to the number of turns and the area enclosed by the loop. More turns means more wire cutting through the magnetic field, increasing voltage. Larger area means more magnetic flux is captured, also increasing voltage. Both factors multiply together - doubling the turns and doubling the area would quadruple the voltage. This is why large, multi-turn loops are used for receiving weak signals. The trade-off is that more turns increase inductance and may require tuning, but the voltage increase is significant.