In Python, the sorted function allows sorting an iterable based on various criteria using the key argument. Select all correct statements regarding the usage of the key argument:

Choice $1$ of $5$:The function provided to the key argument is applied to each item of the iterable to be sorted.

Choice $2$ of $5$:The key argument is always required when calling the sorted function.

Choice $3$ of $5$:The key argument can take a built$-$in Python function like len to sort a list of lists based on length.

Choice $4$ of $5$:The key argument, when used with a custom function, should always return a boolean value to determine the sorting order.

Choice $5$ of $5$:The key argument in the sorted function takes a function that defines the sorting order of the iterable.