C) Win percentage against the next highest placed common opponent in the
standings.

The question is whether this refers to the “next” highest team in the conference, meaning Kansas St - BYU wins the tiebreaker
OR if it means “next” highest team after them, meaning Kansas St doesn’t count as the “next highest team” and it would go down until whichever team BYU lost to, making it so ISU wins.

It really would be a dumb tie-breaker rule if it came down to the next highest team but didn’t count Kansas St, so my guess is it is intended to be the record against the highest team in the conference and BYU would win. Curious to hear your interpretation.

It definitely wouldn’t get to SOS since that is d). It would be resolved at c).