In English mistakes are often made by using the wrong preposition after certain words. This is one example…
To complain about something refers to saying that something is wrong or not good enough.
Lots of people have complained about the noise.
To complain to someone refers to telling someone formally that something is wrong.
If the service was so bad why didn’t you complain to the manager?
To complain of something refers to telling other people that something is making you feel ill or unwell.
She’s been complaining of chest pain recently.
To complain at someone refers to telling someone about something that you are unhappy about, but they are not the source of the complaint and might not be able to do anything about it.
She was complaining at the unfairness of it.
Now what you were looking for….
The correct answer was… B