The word that can substitute the given group of words "A remedy for all ills" is "Panacea."

Explanation of the options:

☀ Panacea: Panacea refers to a remedy or solution that is believed to cure or solve all problems or difficulties.

☀ Libertine: Libertine refers to a person who is morally or sexually unrestrained, often associated with a disregard for conventional behavior or norms.

☀ Convalescent: Convalescent refers to a person who is recovering from an illness or medical treatment.

☀ Rustic: Rustic refers to something that is characteristic of the countryside or rural areas, often associated with simplicity or a lack of sophistication.