Question

Suppose that during any period of ¼ second there is one instant at which the crests or troughs of component waves are exactly in phase and maximum reinforcement occurs. How many beats will occur in 1 second?        A. 8   B. 12   C. 2   D. 4

Answer 1

The correct option is D.
Beat is an interference pattern between two sounds of slightly different frequencies which is perceived as a periodic variation in volume. 
If one beat occur in 1/4 of a second, this means that, a second is divided into four parts and one beat occur in each part. Therefore in a second, there will be four beats.

Answer 2

the correct answer is D.4