If our friends are not able to take us in their car, we must make ...... arrangements to go to the airport.

If our friends are not able to take us in their car, we must make alternative arrangements to go to the airport.
In this context, "alternative" means substitute or backup. The sentence suggests that if the initial plan of having friends take the speaker to the airport falls through, they must make other arrangements. Therefore, "alternative" is the correct choice to complete the sentence, conveying the idea of having a backup plan in case the original plan fails.