A particle of mass $M$ is situated at the centre of spherical shell of same mass and radius $a$. The magnitude of the gravitational potential at a point situated at $a/2$ distance from the centre, will be

$\text{Step 1 - Drawing free body diagram }$

$\text{Step 2 - Net gravitational potential at point P is given by}$

$V_p=\text{(Potential due to sphere)}+\text{(Potential due to particle)}$

As we know that Gravitational potential  due to point mass$=\frac{GM}{r}$

Gravitational potential inside shell $=\frac{GM}{a}$

$\because V_p=\frac{GM}{a}+\frac{GM}{\frac{a}{2}}$

$\Rightarrow V_p=\frac{3GM}{a}$

Hence option (B) is correct.