Everywhere elegant old buildings are being pulled down and mediocre modern structures are being put up.

The sound of running water of stream had a pleasantly sonorous effect on me.

A restaurant that presents spick and span look will naturally attract more customers.

Automobile manufacturers are reviving up to launch a campaign designed to increase consumer awareness about new emission control.

Defection is an unprincipled practice which can do incalculable damage to democratic process.

The police dispersed mob.

It occurred to me that she was incurable.

After advice of his far, he was reconciled to his wife.

The correct option to complete sentence is "momentous." Success gives only a momentous pleasure. The word "momentous" accurately conveys idea of a profound and remarkable pleasure that comes from achieving success. The or options do not fit context of 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 context of positive pleasure.
Option B: "Momentary" means lasting for a very short time. It does not convey enduring and profound pleasure implied by sentence.
Option C: "Monotonous" means dull, repetitive, and lacking variety. It is not appropriate to describe pleasure associated with success.

Therefore, correct choice is "momentous."

The police is on duty.