An outgroup in the evolutionary sense is a clade of organisms that is closely related to the organisms we're interested in (the ingroup), but not more closely related to anything within that ingroup than anything else, "equidistant" from them in an evolutionary sense. The optimal choice for the outgroup is the clade just outside the ingroup.
Killer whales are not an outgroup for mink whales, because they are several clades apart. They aren't closely related enough.
Horses are an outgroup for pigs, because they are very closely related and if we separated the pig clade into specific species all those species would be equidistant from horses in evolutionary terms.
Horses are not an outgroup for pigs and water buffalo, because horses are more closely related to pigs than they are to water buffalo, and thus fail the "equidistant" criterion.
Mink whales are not an outgroup for cows and water buffalo, because they are too distantly related.
Therefore the correct answer is C: Horses are an outgroup for pigs.
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