To measure one wavelength, you would measure the distance between

a. c and a

b. c and e

c. d and b

d. c and f

B. C and E

Explanation:

The wavelength of a wave is the spatial distance between two consecutive points of the wave which have the same shape: for example, the distance between two consecutive crests, or the distance between two consecutive troughs.

Looking at the figure, we see that for this particular wave we can measure the wavelength by taking the distance between :

- the points A and B (two consecutive crests), or

- the points E and F (two consecutive same-shape points), or

- the points C and E (two consecutive same-shape points)

Among the options given, we see therefore that the correct one is  

B. C and E