For any nonnegative real number.

Answer:
A
Step-by-step explanation:
We know that [tex]\sqrt{b}=b^{1/2}[/tex].
We also know that [tex](a^b)^c=a^{bc}[/tex].
We can combine these facts to get [tex](\sqrt{b})^2=(b^{1/2})^2=b^{1/2*2}=b^{1}=b[/tex].
So, the answer is [tex]\boxed{A}[/tex] and we're done!
Note: the non-negative part or the equation comes from the fact that you assume the square root of a negative number doesn't exist. You will change this assumption later in the year, or in further years.