Answer is A.
The equation for profit is Profit $=SP-CP,$
Here, $SP=Q\times 50$ and $CP=5Q^{2}$.
So, when a function attains its maximum value its first order differentiation is zero.
Hence, $50-5\times 2\times Q=0 \therefore Q=5.$
For example :
5 units = $CP = 125 \ SP=250 \ \therefore \text{Profit} =125$
10 units = $CP = 500\ SP=500\therefore \;\text{Profit} =0$ and so forth..
Therefore, its maximum is at unit $= 5$