The correct option is C Monumental
A monumental thing is great in importance, extent, or size. For example, "it's been a monumental effort". Hence, Option D is correct. Among the rest of the options, meaningful means serious, important, or worthwhile, ominous means threateningly inauspicious, evident means clearly seen or understood, rational means based on or in accordance with reason or logic. These options do not express the intended meaning, hence incorrect.