。
She was very surprised by his sudden defiant attitude.
Literal
She [topic-は] his [genitive-の] sudden [adj-の] defiant [adj-的] attitude [at-に] very was-surprised.
A nice modifier chain on 態度 ('attitude'): 彼の (possessive) + 突然の (descriptive) + 反抗的 (descriptive). Note that 突然 is a 'no-adjective' — it modifies nouns via の rather than な (突然の出来事, 'sudden event'). 反抗的 uses the ~的 suffix ('-tic, -ish') to convert the noun 反抗 ('resistance, rebellion') into a na-adjective: 反抗的な ('rebellious, defiant'). The に marks the source of the surprise. びっくりする ('be surprised') is colloquial; 驚く is the more formal alternative.