The universe cooled after the big bang. at some point, hydrogen atoms combined to form helium. what is this process called? a) fusion b) fission c) red shift d) nuclear decay

fusion

Explanation:

Nuclear fusion is a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.

Nuclear fission is a process which involves the conversion of a heavier nuclei into two or more small and stable nuclei along with the release of energy.

Red shift is a phenomenon where electromagnetic radiation from an object undergoes an increase in wavelength.

Nuclear decay is a type of process in which a less stable nuclei decomposes to a stable nuclei by releasing some radiations or particles like alpha, beta particles or gamma-radiations.