During the course of the 16th century for the underpaid musicians provided a good source of supplementary income.

a. printing
b. word painting
c. proofreading
d. spying

The correct answer is actually D - Spying.

The reason why D is the correct answer is because during those times, musicians could be invited to parties, balls, and festive occasions that were hosted by one family or another family or nobility or any other important group of people who could pay them a hefty sum to tell them what another family was doing and vice versa. That is why their job offered them the opportunity to work as part-time spies.