Of outstanding significance

A. Monumental

B. Rational

C. Ominous

D. Evident

Answer: Option A

Solution(By Examveda Team)

One word substitute is Monumental.

Monumental: great in importance, extent, or size.
Rational: based on or in accordance with reason or logic.
Ominous: giving the worrying impression that something bad is going to happen.
Evident: clearly seen or understood.