Final answer:
An automatic transfer from a savings account is usually the most cost-effective form of overdraft protection, as it can prevent costly overdraft fees by using funds from your own savings without extra charges.
Step-by-step explanation:
The most cost-effective option for overdraft protection is typically an automatic transfer from a savings account. This option links your checking account to your savings account and automatically transfers funds if the checking account is overdrawn, thus preventing an overdraft fee. In contrast, a line of credit or an overdraft protection plan may involve fees or interest charges, whereas an overdraft fee is a punitive charge imposed when you spend more than what is available in your checking account. These fees can be costly and thus not cost-effective. Bank accounts, both checking and savings, provide a secure and liquid way to manage your finance.