Topic$($s$)$: Fields $($ahl$)$

Four positive charges are fixed at the corners of a square that has a diagonal length of $d.$ Three the charges have a charge of $+Q.$

The total electric potential at the centre of the square is $\frac{10kQ}{d}.$

What is the magnitude of the fourth charge?

A$.Q$

B$.Q\sqrt[\phantom{2}]{2}$

C$.2Q$

D$.4Q$