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 = $CP = 125 \ SP=250 \ \therefore Profit=125$
10 units = $CP = 500\ SP=500\therefore Profit=0$ and so forth..
Therefore its maximum at unit $= 5$