Solution:

Substituting K= 1, will help us to easily solve this question
F= $\frac{1}{{3k}^3}$=$\frac{1}{27}$

Now, when K is halved, 1 will become 1/2
Thus, we get $\frac{1}{{(3*1/2)}^3}$=$\frac{ 1}{27/8}$= $\frac{8}{27}$

Thus for the original value of F this value has 8 in the numerator, which we get if we multiply F by 8
i.e. 8F$= \frac{1}{{(3K/2)}^3}$

IMO B

Hope this helps!