Very good question. The rule (from MLB) is below.
Since starter went 5 (in HS this would be 4), I don't think 10.17 (b) would apply since the starter went at least 4. Thus you get into rule 10.17 (c). By the letter of the rule, the appearance of (B) was not 'brief' because it was one inning. You could certainly argue it was very ineffective, because of the 6 runs. I think a scorer who spent any time thinking about this (which won't happen since the visiting team won) would probably give the win to C for being the most effective, although B could also be given the win and C the save. I'll post another case that happened to my son last night after this post.
10.17 WINNING AND LOSING PITCHER
(a) The official scorer shall credit as the winning pitcher that pitcher whose team assumes a lead while such pitcher is in the game, or during the inning on offense in which such pitcher is removed from the game, and does not relinquish such lead, unless
(1) such pitcher is a starting pitcher and Rule 10.17(b) applies; or
(2) Rule 10.17(c) applies.
Rule 10.17(a) Comment: Whenever the score is tied, the game becomes a new contest insofar as the winning pitcher is concerned. Once the opposing team assumes the lead, all pitchers who have pitched up to that point and have been replaced are excluded from being credited with the victory. If the pitcher against whose pitching the opposing team gained the lead continues to pitch until his team regains the lead, which it holds to the finish of the game, that pitcher shall be the winning pitcher.
(b) If the pitcher whose team assumes a lead while such pitcher is in the game, or during the inning on offense in which such pitcher is removed from the game, and does not relinquish such lead, is a starting pitcher who has not completed
(1) five innings of a game that lasts six or more innings on defense, or
(2) four innings of a game that lasts five innings on defense,
then the official scorer shall credit as the winning pitcher the relief pitcher, if there is only one relief pitcher, or the relief pitcher who, in the official scorer’s judgment was the most effective, if there is more than one relief pitcher.
Rule 10.17(b) Comment: It is the intent of Rule 10.17(b) that a relief pitcher pitch at least one complete inning or pitch when a crucial out is made, within the context of the game (including the score), in order to be credited as the winning pitcher. If the first relief pitcher pitches effectively, the official scorer should not presumptively credit that pitcher with the win, because the rule requires that the win be credited to the pitcher who was the most effective, and a subsequent relief pitcher may have been most effective. The official scorer, in determining which relief pitcher was the most effective, should consider the number of runs, earned runs and base runners given up by each relief pitcher and the context of the game at the time of each relief pitcher’s appearance. If two or more relief pitchers were similarly effective, the official scorer should give the presumption to the earlier pitcher as the winning pitcher.
(c) The official scorer shall not credit as the winning pitcher a relief pitcher who is ineffective in a brief appearance, when at least one succeeding relief pitcher pitches effectively in helping his team maintain its lead. In such a case, the official scorer shall credit as the winning pitcher the succeeding relief pitcher who was most effective, in the judgment of the official scorer.
Rule 10.17(c) Comment: The official scorer generally should, but is not required to, consider the appearance of a relief pitcher to be ineffective and brief if such relief pitcher pitches less than one inning and allows two or more earned runs to score (even if such runs are charged to a previous pitcher). Rule 10.17(b) Comment provides guidance on choosing the winning pitcher from among several succeeding relief pitchers.