"Your pants are nice."
Translation:Ton pantalon est joli.
When meaning a whole collection of pants I think we are to use (in real life, not on Duolingo) the plural. But here we are to avoid the plural in order to learn (and show Duo that we understand) that pants/jeans/trousers/shorts and similar leg garments are singular in French while plural in English.
"Pants" are considered plural in English, but not in French. Really you're only talking about 1 article of clothing, but in English we still call them a "pair of pants" for some reason. You would translate "my [pair of] pants" = "mon pantalon" to imply a single pair of pants.
In French, the singular word is "le pantalon" and the plural form is "les pantalons". In English, the words pants/trousers may be either singular or plural but they are always treated as grammatically plural unless you say "a pair of pants/trousers" which is grammatically singular because of saying "a pair of".
No. Pantalon refers to trousers (in AmE, it may be either "pants" or "trousers"). We recognize that "pants" in BrE refers to underpants (un slip), not trousers. For some reason, that particular word is not included in Duo's vocabulary. Perhaps they are afraid it will start a firestorm of snickering comments. We will never know, I'm afraid.
There is no "sont joli" (ever). You may say "tes pantalons sont jolis", "ton pantalon est joli", "votre pantalon est joli" or "vos pantalons sont jolis", but not "ton/votre pantalon sont joli". Do you see why? "Pantalon" is singular in French and requires "est". But the English word "pants/trousers" is always grammatically plural even when it refers to a single item.