The numerator of our fraction is the "cube root of 7" which can be expressed as 7^(1/3). In the same manner, our denominator is "fifth root of 7" which is equal to 7^(1/5).
The mathematical expression of the given is,
s = 7^(1/3) / 7^(1/5)
Since the numbers have the same bases, it can also become,
s = 7^(1/3 - 1/5)
The simplification of the operation above is,
s = 7^(2/15)
Hence, the answer to this item is letter D.