Answer
We can rank the situations according to the magnitude of the gravitational force on particle P due to the shell:
$(b) = (c) \gt (a)$
Work Step by Step
In the Review & Summary on page 376, the text states, "...if either of the bodies is a uniform spherical shell or a spherically symmetric solid, the net gravitational force it exerts on an external object may be computed as if all the mass of the shell or body were located at its center."
In situation (a), the point P is located inside the spherical shell. Therefore the net gravitational force exerted on particle P is zero.
In situation (b) and (c), we can imagine that all the mass $M$ is located at the center of the spherical shell. Since the distance to point P is equal and the mass is equal in both situations, the net gravitational force exerted on particle P is equal in both situation (b) and situation (c).
We can rank the situations according to the magnitude of the gravitational force on particle P due to the shell:
$(b) = (c) \gt (a)$
