What is the plurial of -
Tu peux monter par l'ascenseur.
The question is asking for the plural form of the sentence "Tu peux monter par l'ascenseur." In French, the plural form of this sentence would be:
Vous pouvez monter par les ascenseurs.
To form the plural, the subject pronoun "tu" changes to "vous" (which can mean either "you" singular or "you" plural), and the article "l'" (before "ascenseur") changes to "les" to indicate multiple elevators. The verb "peux" also changes to "pouvez" to agree with the plural subject pronoun.