Medicine which lessens pain

Answer: Option C

One word substitute is Anodyne.

Balm: a fragrant cream or liquid used to heal or soothe the skin.
Panacea: a solution or remedy for all difficulties or diseases.
Anodyne: a painkilling drug or medicine.
Antibiotic: a medicine (such as penicillin or its derivatives) that inhibits the growth of or destroys microorganisms.