Accretion is defined as:

A. Gravity that pushes matter in a circular manner into an outer source.

B. The disk of material surrounding a black hole.

C. Gravity that pulls matter inward in a spiraling matter.

D. None of the above.

Answer: C. Gravity that pulls matter inward in a spiraling matter.

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.