**Answer: option b.**

**Explanation:**

The kinetic energy of a spring with constant K is calculated as:

kinetic energy = (k/2)*x^2

Where x^2 is the displacement of the spring with respect to it's rest position.

This can be written as a function like:

x = A*cos(2*pi*f*t)

where:

A is the amplitude (the maximum distance that the spring can move in each direction)

f is the frequency (and 2*pi*f is the angular frequency)

and t is the variable, it represents the time.

Replacing this in the kinetic energy equation, we get:

kinetic energy = (k/2)*(A*cos(2*pi*f*t))^2

This is the same as the option b: b. 1/2kA^2cos^2(2πft)

Then the corrrect option is b.