Well, the relationship between the two isn't parasitism. In parasitism, the parasite invades the host and benefits from it while the host suffers (and sometimes dies.) This relationship is very one-sided, and that's not what's happening in a cow's stomachs.
It also is not commensalism. In this relationship, one organism benefits while nothing bad or good happens to the other. An example would be seagulls at the county fair. They eat the popcorn that gets dropped, which benefits them, but they don't hurt people (unless they poop on them by accident : )
That leaves us with the correct answer, mutualism. In mutualism, both organisms benefit from the relationship. The bacteria living in the cow's stomachs benefit because they have a steady supply of nutrients coming in and a hospitable place to eat them. The bacteria, in turn, produce cellulase, which helps the cows with their digestion.