。
She's practicing the violin right now.
Literal
She [topic-は] now violin 's practice [object-を] is-doing.
バイオリン is the standard katakana for violin; you'll occasionally see ヴァイオリン (with the rare ヴ-katakana) in classical-music journalism, signaling a slightly more cosmopolitan flavor. 練習をする pairs the activity noun 練習 ('practice') with the light verb する; the alternative 練習する fuses them into a suru-verb. の binds 'violin's practice' = 'violin practice' as a compound — a productive way to make 'X-ing of Y' phrases. ~しています is the polite ongoing form.