Learn French: Certains repas ont un supplément, lesquels ? - Some meals have an extra charge, which ones?

Quels is an interrogative adjective, meaning it must sit directly next to a noun (as in quels repas). Because we are dropping the noun entirely to ask "which ones?", we must use an interrogative pronoun instead. Since it refers back to repas, which is masculine and plural, we use lesquels.

How can you tell repas is plural in this sentence? Doesn't the singular form also end in an "s"?

Yes, un repas ends with an "s" even in the singular! You have to rely on the surrounding words for clues. In this sentence, the adjective certains (some) is plural, and the verb ont (they have) is the plural form of avoir.

Does un supplément specifically mean a monetary extra charge here?

Exactly. While it literally translates to a "supplement" or "addition", in the context of menus, restaurants, and hospitality, un supplément is the standard French term for "an extra charge" or "an additional fee".

Why is there a space between lesquels and the question mark?

In French typography, any punctuation mark that is made of two parts (like ?, !, :, and ;) requires a space immediately before it.