She wore, _______ was very uncommon in the country, a red garment.

what
She wore a red garment , what was very uncommon in the country.
后面的从句,来补充说明 a red garment ,而what就是代 a red garment 的

which
She wore a red garment ，which was very uncommon in the country.
这句话的前一句是一个完整的句子，而后一句是对前一句的补充。
她穿了一件红色的衣服，这件红色的衣服在这个国家很不普通。
which代指a red garment

which
这里的句子可以改为：She wore a red garment,___was very uncommon in the country.后面的是对red garment的补充，因为有了逗号，所以构成一句非限定性定语从句，不用what，而用which 引导（that也不行）。这是高中的语法要点。

which