Since relative fitness depends on relative contribution to the next generation, a sterile (cannot reproduce) mule will contribute nothing and have zero for relative fitness.
Work Step by Step
Review the concept of relative fitness as described in the textbook, and then see that sterility = no offspring/input into the next generation. Plug that into the definition, and it leads to a relative fitness of zero. The rest of the population, which would be the asses and horses nearby, would be able to reproduce.