Susie is participating in research that assesses the strength of her bond with her partner. as part of this research, she is asked, "all things considered, how attractive are the people other than your partner with whom you could become involved? " which element of the commitment model of relationships is this question measuring?

I believe the answer is: Alternative partners

Alternative partners refers to the characteristic that measured how strong the commitment you're giving to maintain your relationship with your partner.
Stronger commitment is shown in partners who stay loyal with their partner despite being surrounded by other attractive potential mate.