is the force per unit mass on a small (test) mass at the point. (d)

$$g = \frac{F}{m}=\frac{G\frac{Mm}{r^2}}{m}=G \frac{M}{r^2}$$.

Its unit is Nkg^{-1}. It is a vector quantity and is always directed towards the centre of the mass and points tangentially ($$\perp$$) to its surface.

To compare the gravitational field strength for different masses/planets

$$\frac{g_x}{g_y}=\frac{M_x}{M_y} \left(\frac{r_y}{r_x}\right)^2$$.