。
She put on an innocent face.
Literal
She [topic-は] [unaware-そ知らぬ] face [object-を] did.
そ知らぬ顔 (so-shiranu kao) is a fixed expression: そ (a phonetic emphasizer/intensifier) + 知らぬ (the classical negative attributive of 知る 'know') + 顔 ('face'). Literally 'unknowing face,' meaning 'a feigned look of ignorance, a poker face, a deliberately innocent expression.' Used when someone pretends not to notice or know something. Variants include 素知らぬ顔 with the kanji 素 ('plain, bare'), reinforcing the 'plain unawareness' image. The pattern N+の/な+顔をする ('make an X face') is highly productive: 嫌な顔をする ('look annoyed'), 困った顔をする ('look troubled'), 何食わぬ顔 ('without batting an eye').