One-word substitution (degree of importance): Identify the adjective that means of outstanding significance, exceptionally great in scope, scale, or impact. Context: Avoid words suggesting mere evidence, ominous foreboding, or rationality rather than importance. Instruction: Choose the best single-word substitute.

Given data

• Sought meaning: of outstanding significance.
• Options: Monumental, Rational, Ominous, Evident.

Concept / Approach
Monumental conveys exceptional size/importance/impact — figuratively ‘‘worthy of a monument.’’

Elimination
Monumental — outstanding significance; correct. ✅Rational — based on reason; not degree of importance. ❌Ominous — portending evil; tone, not significance. ❌Evident — obvious; clarity, not magnitude. ❌

Usage
‘‘A monumental discovery changed cancer therapy.’’

Final Answer
Monumental