。
She took responsibility for the plan.
Literal
She [topic-は] that plan's responsibility [object-を] took.
責任を取る ('to take responsibility') is one of those phrases worth memorizing as a fixed unit — not just for daily speech but because it has weighty cultural resonance in Japanese workplaces, where shouldering blame can mean stepping down or formally apologizing. The high-profile 引責辞任 ('resignation to take responsibility') headlines, where executives quit over scandals, is the visible institutional version of the same idiom. Grammatically, ~の責任 is the genitive 'the responsibility of/for X', and を marks 責任 as the thing being taken.