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.

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.

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