。
She put down the pen.
Literal
She [topic-は] pen [object-を] set-down.
おく ('to put, place, set down') is the everyday verb for setting an object down — written in hiragana here, though the kanji is 置く. Putting a pen down is a small gesture but a recurring one in narrative scenes (interview ending, finishing writing, pause in thought). The plain past おいた is the simplest possible past-tense rendering.