"Her pants"

Sus.... Pantalones is plural

Yes the "de" is because of possession

No:

• She= ella
• Her=Sus, su (de ella)
• His=Sus, su (de él)

So

• Ella lleva puestos sus pantalones= She wear her pants

In English we add an 's' to a pronoun to show possession, like Elise's cat, but in Spanish the only way to show possessiveness is "el _ de ella" or a possessive article, which go my: mi(s) your: tu(s) her/his: su(s) our: nuestro(s) y'alls (in spain): vuestro(s) theirs, y'alls (in mexico): su(s)