Answer: C. Gravity that pulls matter inward in a spiraling matter.
Step-by-step explanation:
In astrophysics, particles are pulled towards a massive object due to gravity which further attracts more particles. A disk of particles is formed around the object. Hence, we can define Accretion as a process in which particles are accumulated in a form of a disk of surrounding a massive object (like a black-hole) due to gravity which pulls the matter inwards and matter spirals around the massive object. Galaxies, planets etc. are formed out of accretion process. Thus, the correct option is C.