Learn French: Il y a trois excursions, lesquelles sont incluses ? - There are three excursions, which ones are included?

lesquelles means "which ones" and must match the gender and number of the noun it replaces. Here, it refers to trois excursions. Because the word excursion is feminine (une excursion) and plural (there are three), we need the feminine plural form lesquelles.

Can I just say quelles sont incluses instead of using lesquelles?

No, because quelles is a question word that must be attached directly to a noun, like "which excursions" (quelles excursions). When you want to drop the noun and just ask "which ones", you must use a pronoun like lesquelles.

Why is incluses spelled with an -es at the end?

The word inclus acts as an adjective here, so it has to agree with the subject of the sentence, which is lesquelles (referring to the feminine plural excursions). The masculine singular is inclus, so we add an -e for feminine and an -s for plural, making it incluses.