To measure one wavelength, you would measure the distance between

a) c and a
b) d and b
c) c and e
d) c and f

The answer is C) C and E. 

The wavelength, or the period of the wave, is the length or amount of time of one complete cycle, meaning the time it takes for the wave to return to its original position. A period involves one trough, one crest, and a return to rest, or the zero. We can see that this occurs between C and E. 
Another way to look at it is the period of time, or length, before the wave returns to its original position. Where the measurement starts isn't really relevant, as long as the measurement is where the wave returns to that position.