The correct option is D sprightliness
Italicized word 'Agility' means ability to move quickly. The meaning of this word is option D, 'Sprightliness'. It is quality of being energetic and healthy. The context of the sentence is that the athlete's quality of being energetic was appreciated by the spectators. Option A, grace means elegance and charm. Option B 'strength' means power. Option C 'Alertness' means aware and attentive. None if these options, A, B and C are correct to be replaced for agility. So, the answer is Option D, Sprightliness.