。
She realized that the question was difficult to answer.
Literal
She [topic-は] that question [target-に] answer [nominalizer-の] [subject-が] difficult [quotative-と] understood.
A multi-clause sentence with two embeddings. Inside, 答えるのが難しい ('answering is difficult') uses the nominalizer の to turn 答える into something が-markable, paired with the い-adjective 難しい. Outside, ~とわかる ('come to understand that') wraps the whole inner clause via quotative と. The に in 質問に答える marks the target — answering directed at the question. The whole captures the moment of recognizing the difficulty rather than asserting it from the start.