The advertisement assured the public that the medicine would give back to the users, their youthful vigour and appearance.

Rejuvenate
Restore
Replenish
Render

The correct answer is A. Rejuvenate.

Rejuvenate means to make someone or something look or feel younger. It can also mean to make something new or fresh again.

Restore means to bring something back to its original condition. It can also mean to return something to its rightful owner.

Replenish means to make something full again. It can also mean to provide someone with something they need.

Render means to give or produce something. It can also mean to interpret something in a particular way.

In the context of the advertisement, the medicine is said to give back to the users their youthful vigour and appearance. This means that the medicine is supposed to make the users look and feel younger. The word that best describes this is rejuvenate.