she's wearing a strapless bra, it kinda sucks/holds on so that you can't see the nipples but you can't see the straps in the back. obviously underwear who'd wear denim without underwear? that would be uncomfortable
She is wearing a bra because of shape and support. Also I guess a thong because going commando in those shorts would be too revealing and as others have said denim too rough.
The only opinion from girls was selected the Most Helpful Opinion, but you can still contribute by sharing an opinion!