The weather is so changeable.I don't know______

选C.

I don't know what to wear. “what to wear”做的I don't know的宾语，“what”做的是“wear”的宾语，因为wear是及物动词，是必需要带宾语的，如：wear a hat/a pair of shoes/glasses。

我觉得因为那里所要抉择的是衣物格式,因为天冷穿厚,天热穿薄,根本不存在及物动词的概念,只是常理性的揣度罢了.

A，due to my perception

The weather is so changeable.I don't know(C.what to wear)