Option 1 -> consequential means important or significant, which is a synonym of momentous, not an antonym.
Option 2 -> insignificant means not important or trivial, which is the direct opposite of momentous.
Option 3 -> pivotal means crucial or very important, which is also a synonym of momentous.
Option 4 -> mediocre means average quality, which relates to quality rather than importance or significance.
Hence, Option 2: insignificant -> "Momentous" means very important or significant, and its direct antonym is "insignificant" which means unimportant or trivial -> correct