Star formation begins when a. gas and dust collapse inward. c. the fusion of hydrogen slows down. b. nuclear fusion starts in the core. d. the core becomes carbon and oxygen.

The correct answer is option a, that is, gas and dust collapse inward.  

Within the formation of the stars of comparatively dense concentrations of interstellar gas and dust takes place called molecular clouds. These regions are tremendously cold, and at these temperatures, gases turn molecular, that is, the combination of atoms takes place. This deep cold makes the gas to combine to high densities, and when a certain point of density reaches, the formation of the star takes place.  

The formation of star initiates when the denser segments of the cloud core collapse under the influence of their own gravity or weight. These cores usually exhibit masses of about 10^4 solar masses in the form of dust and gas.