In astrophysics, the "Roche limit" defines:
a) The distance within which a celestial body, held together by its own gravity, will be torn apart by tidal forces from a larger body.
b) The maximum distance a star can be from a galaxy before being gravitationally bound to it.
c) The boundary beyond which a planet's atmosphere cannot support life.
d) The distance a comet travels within a solar system before being captured by a star.