。
She was transferred from headquarters to a branch office last month.
Literal
She [topic-は] last-month headquarters [from-から] branch-office [to-へ] was-made-to-transfer.
させられた is the causative-passive form — 転勤する (transfer) → 転勤させる (make someone transfer) → 転勤させられる (be made to transfer). This double stacking implies the transfer was not her choice; she was assigned or forced to move. 本社 (ほんしゃ, headquarters) and 支社 (ししゃ, branch office) are standard business vocabulary. The から...へ pair marks the start and end points of the movement.