一道英语选择题，求详细解答并翻译

这是 see to sth 处理某事 的被动语态
主动语态是 they would see to his complaint

变成被动语态即 his complaint would be seen to.

see to是动介词组 变成被动语态后 to 不能少
所以 答案是 A. to

其它选项 均不是与see搭配的 固定词组 所以不选
句子意思 经理向顾客保证他的投诉将被立刻处理

选A, to, 因为see to是一固定短语，解决，处理。句子为The manager 是主语assured 是谓语the customer是间接宾语 that his complaint would be seen ___ immediately. 是从句作的直接宾语。
句子意思;
经理向顾客保证他们的投诉将被立刻决。

这里是用了被动语态的倒装，如果改成正常语序就很容易理解了：
The customer's complaint would to be seen immediately.
所以这里就是填写介词to哦， 选择A
大意：经理向顾客保证他的投诉会马上被处理的。

宾语补足语，经理保证顾客的投诉将会立即受理，应该选择at吧，祝好运