What came before the big bang is one of the primary unanswered questions about the origin of
galaxies
life
earth
the universe
the solar system

The big bang theory describes the origin of the Universe - so the theory of the Big Bang, together with what came before it is a part of the study about the Universe- so the correct answer is "the universe".

Some people hypothesize that there was another universe before the Big Bang, and some that there was nothing.

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).