He was ---- to go out with his girl friend.

C很明显的,let( let, let)没有letted这种形式.
let 的用法
1）当let后只有一个单音节动词,变被动语态时,可用不带to 的不定式.例如：
They let the strange go.他们放陌生人走了.
---> The strange was let go.
2）当let 后宾补较长时,let 通常不用被动语态,而用allow或permit 代替.例如：
The nurse let me go to see my classmate in the hospital. 那护士让我去探望住院的同学.
----> I was allowed / permitted to see my classmate in the hospital.