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

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.

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