How much mass should be attached to a vertical ideal spring having a spring constant (force constant) of 39.5 n/m so that it will oscillate at 1.00 hz?

The frequency of a simple harmonic oscillator such as a spring-mass system is given by

where 
k is the spring constant
m is the mass attached to the spring.

Re-arranging the formula, we get:

and since we know the constant of the spring:

and the frequency of oscillation:
f=1.00 Hz
we can find the value of the mass attached to it:

Ithink d is the right answer. hope this !