Final answer:
The term "big bang" refers to the origin of the universe, theorized as a rapid expansion of space approximately 13.7 billion years ago, evidence for which includes the cosmic microwave background radiation. Therefore, correct option is a.
Step-by-step explanation:
The term "big bang" refers to the origin of the universe. This theory posits that approximately 13.7 billion years ago, the universe began not as a traditional explosion scattering matter into preexisting space, but as a rapid expansion of space itself, in which space, time, matter, and energy were all created.
Following the Big Bang, the universe has continued to expand and cool, leading to the formation of galaxies within the remnants of this primordial fireball.
Evidence for the Big Bang includes the observed expansion of the universe, the cosmic microwave background radiation (CMB), which is the redshifted afterglow of the event, and is measured as coming from all directions in space.
This CMB is the most perfect blackbody radiation spectrum ever detected and indicative of the Big Bang's expansion-cooled temperature of 2.725 K.