The correct option is
A momentous
Option D: 'Momentous' means 'of great importance or significance'. It relates to the given sentence: the pleasure achieved from success is of great importance.
The sentence formed is meaningful and hence option D is correct.
Option A: 'Mordant' means 'having or showing a sharp or critical quality', 'biting'. This word expresses a quality (of a human) in a negative way. It cannot describe 'success'.
Option B: 'Morbid' means 'having an unhealthy interest in the horrors of life, like death and disease'.
Option C: 'Monotonous' means 'dull, tedious and repetitive'.
These words do not form a complete meaning here.
Hence options A, B and C are wrong.