(N/A) No. Gravitational influence cannot be shielded because gravity is independent of the nature of the medium and the presence of other matter.
$(b)$ Yes. If the space station is large,the astronaut can detect the variation in the gravitational field (tidal forces) across the dimensions of the station.
$(c)$ The tidal effect depends on the gradient of the gravitational field,which is proportional to $1/r^3$,whereas the gravitational force itself is proportional to $1/r^2$. Since the Moon is much closer to the Earth than the Sun,the gradient of the Moon's gravitational field is larger,resulting in a greater tidal effect.