If a string vibrates at the fundamental frequency of 528 hz and also produces an overtone with a frequency of 1,056 hz, this overtone is the
a. third harmonic.
b. second harmonic.
c. fourth harmonic.

So we wan't to know which harmonic is the frequency of Fn=1056 Hz if we know that the fundamental frequency F1=528 Hz. So the frequency of n-th harmonic Fn is fundamental frequency F1 multiplied by n or Fn=n*F1. In our example we know the fundamental frequency and an overtone frequency and we want to know which harmonic it is so we are looking for n. So n=Fn/F1 and after we input our numbers we get that n=1056/528=2. So the frequency of 1056 is the frequency of the second harmonic.