The________were tied behind his back.

driver's hands

The_driver's hands_were tied behind his back.

driver
driver hand
driver's hand
driver's hands
我是从事英语的，不明再问。

driver 不符合实际
driver hand 表达错误，在英语语法中没有这样的表达
driver's hand 因为后面的be动词是复数，所以应该是“hands”
driver's hands 真确答案

driver's hands因为是双手