If I remember correctly from orgo I, I think it's because KMnO4 is not specified as hot and concentrated. When it is cold and reacted with a base, it will form syn diols. If KMnO4 and H+ were to react with added heat, it would cleave and the resulting product would not be hexanoic acid. Although the question didn't specify conditions, I don't believe you would get hexanoic acid either way. Hopefully this helps. Someone please correct me if I'm wrong.
@hellocakes appears to be correct. C will produce two carboxylic acid groups on opposite ends of an alkyl chain, which would not be hexanoic acid.
The mechanism for D is that KMnO4 just oxidizes the hell out of the alkyne group. KMnO4 is a very strong oxidizing agent and appears to generally always progress to COOH if appropriate substrate material is available.