A mortgage loan originator $($ MLO$)$ originates a $5/1$ ARM where the indexed rate is likely to be higher than the introductory rate. The Truth in lending act$($TILA$)$ states that an MLO must calculate a borrower$$s payment amount on which of the following?

Payment amount during the fixed introductory period.

An average of the varying payment amounts over the life th eloan.

The total amount of the payments.

Fully indexed rate of the loan.