Answer
B. Wave A has the higher frequency. Its frequency is 25 Hz.
Explanation
Frequency is the number of oscillation/revolutions in a unit time. On the other hand, period is the time taken to complete one oscillation/revolution.
From the definition above we can conclude that, frequency(f) is the reciprocal of period(T). That is;
f = 1/T and
T = 1/f
Wave A
f = 1/(4/100)
= 100/4
= 25 Hz
Wave B
f = 1/(1/7)
= 7/1
= 7 Hz
So, wave A has the higher frequency. Its frequency is 25 Hz.