Success gives only . . . . . . . pleasure.

The correct option to complete the sentence is "momentous." Success gives only a momentous pleasure. The word "momentous" accurately conveys the idea of a profound and remarkable pleasure that comes from achieving success. The other options do not fit the context of the sentence as well:

Option A: "Mordant" means having a sharp or critical quality, often used in a negative or sarcastic sense. It does not fit the context of positive pleasure.
Option B: "Momentary" means lasting for a very short time. It does not convey the enduring and profound pleasure implied by the sentence.
Option C: "Monotonous" means dull, repetitive, and lacking variety. It is not appropriate to describe the pleasure associated with success.

Therefore, the correct choice is "momentous."