。
She picked the red dress.
Literal
She [topic-は] that red dress [object-を] chose.
A simple structural sentence showing the い-adjective 赤い in its attributive (modifying) form, attached directly to the following noun. The noun ドレス itself is a katakana loanword from English, used in Japanese specifically for Western-style dresses — kimono, yukata, and other traditional garments have their own dedicated names, not folded under ドレス.