• A Hz is just an inverse second ie 1/s or s^-1. A vibration is a unitless quantity. The queation has given us 20 vibrations in 2.5 seconds or 20/2.5 * 1/s = 20/2.5 Hz =8Hz.

    If that corresponds to option A or option B who knows. But i would guess whoever made the test put the answers in in the order they are written without realising the testing software shuffles the options when askibg students. Hence the button with 8Hz or B is the correct answer.