。
She laughed when she saw his hat.
Literal
She [topic-は] his [genitive-の] hat [object-を] saw-and laughed.
The te-form 見て links the cause (seeing the hat) to the effect (laughing). Without an explicit 'because' marker like から or ので, the te-form alone carries the cause-and-effect reading naturally. 笑う ('laugh') here implies amused/surprised laughter at the hat's appearance — context fills in what isn't stated.