楼主是漏打了前面的主句吧~
我假设全句是：The cyclist holds on his raincloth ____the wind could't blow it away.
这里用so意思是对的~
但是,语法上是不对的.如果要用so的话,这句话要分为两主句：
The cyclist holds on his raincloth .So the wind could't blow it away.
而这题是一句话,只能用so that.
主句为The cyclist holds on his raincloth,而so that 后面引了一个从句the wind could't blow it away.
希望能对楼主有所帮助~O(∩_∩)O~