Final answer:
Phobos is the satellite of Mars that is a likely captured asteroid and has an unstable orbit predicted to lead to a collision with Mars.
Step-by-step explanation:
The satellite of Mars described as a likely captured asteroid with an unstable decaying orbit that will eventually collide with Mars is d. Phobos. Phobos and its smaller sibling Deimos are believed to be captured asteroids due to their irregular shapes and heavily cratered surfaces which resemble other smaller asteroids in the Solar System. These Martian moons were first studied at close range by the Viking orbiters in 1977.