The boy -----in white is a new student from Suzhou.

A 与“in”连用时,意思是穿着,表示一种状态
B 也表示状态,后边直接跟衣服,不跟介词.如果选B,句子应该为：The boy wearing white clothes is a new student from Suzhou.
C 表示穿衣的动作,不能表示状态,在此句中不适用.“put on”例句：Please put on your coat.It's very cold outside.这里强调的是动作.
D 是名词“衣服”,
故选A