Answer: b. backup offer
Step-by-step explanation:
A backup offer is one that is made when an offer has already been made by another. With a backup offer, the person offering it is acknowledging that someone else has made another offer that was accepted but they still offer this in case the accepted offer falls through for whatever reason.
If the seller accepts this offer, they will have a contract with the person offering that legally obliges them to sell the good in question to the person offering if the current offer is not honored.