The "ringing sound" are actually the soundwaves travelling from the phone to your ears.
Wavelength increases as Frequency decreases and vice versa.
This is because Wavelength and frequency are inversely related.
I think it is d I hope this help you if not let me know if it is not right
Given:
The force of attraction is F = 48.1 N
The separation between the charges is

Also, the magnitude of charge q1 = q2 = q.
To find the magnitude of charge.
Explanation:
The magnitude of charge can be calculated by the formula

Here, k is the Coulomb's constant whose value is

On substituting the values, the magnitude of charge will be

Thus, the magnitude of each charge is 9.91 x 10^(-4) micro Coulombs.