What is the verb for complaint?

The verb of complaint is complain.
Complains, complaining and complained are also verbs.

Here are some sentence examples:
"I want to complain to the manager".
"She complains about the sock she found in her soup".
"The manager had no idea why she was complaining, the sock added flavour".
"The manager told the chef that the lady complained about his speciality sweaty foot soup".