。
She was arrested by the police.
Literal
She [topic-は] police [by-に] arrest-was-done.
A textbook direct passive: the subject (彼女) undergoes the action (逮捕する → 逮捕される), and the agent (警察) is marked with に. する verbs form their passive as ~される, so 逮捕する becomes 逮捕される. The に here is the standard agent-of-passive marker — distinct from the more formal によって, which would sound more bureaucratic and is often used in news headlines.