The old man shows no sign of infirmity even though he is eighty years old.

Answer: Option C

Indolence : avoidance of activity or exertion; laziness.
Fickleness : changeability, especially as regards one's loyalties or affections.
Feebleness : a state of being very weak.
Firmness : the quality of having a solid, almost unyielding surface or structure.
Infirmity : physical or mental weakness.

Synonym of Infirmity is Feebleness