When users do not enter the amount they paid, the system automatically assumes they paid nothing. This functionality is an example of a
O default argument.
O subroutine.
O keyword argument.
O return statement.

When users do not enter the amount they paid, the system automatically assumes they paid nothing. This functionality is default argument.

Explanation:

A default argument is a value provided in a function declaration that is automatically assigned by the compiler if the caller of the function doesn't provide a value for the argument with a default value.