I used to skate quite well but I have lost the knack now.

Answer: Option C

Agility : ability to move quickly and easily.
Strength : the quality or state of being physically strong.
Skill : the ability to do something well; expertise.
Stamina : the ability to sustain prolonged physical or mental effort.
Knack : an acquired or natural skill at doing something.

Synonym of Knack is Skill