What does "customer" mean in this problem?

Hello! It isn’t quite evident for me in the task what the function should return: just a name of the best customer or the whole row in the list of lists!?

It should be the whole row. At the start of the screen we read:

In this problem, the input is, list_of_customers — a list of lists where each of its elements represents the best customer of a given day in a popular online store.

So the customer is the whole row.