The manager assured the customer that his complaint would be seen __A__ immediately.(A) to (B) at (C) on (D) with

see to sth,照料，负责

see to 是一个词组，是“处理”的意思

这句里be seen to意思是 被处理，被关注

see to something/someone

to deal with a person or task that needs to be dealt with or is waiting to be dealt with
处理，办理，应付

"These letters need posting." "I'll see to them later."
“这些信需要寄出去。”“我过一会儿来处理。”
Mrs Chapman asked for some help with the orders - could you see to it?
查普曼夫人在订货方面需要帮助——你能办一下吗？
[ + that ] Please see to it that no one comes in without identification.
请注意不要让没有身份证明的人进入。