I have a horse. Do you know what color it is? Allan said, "I guess it is not black". Brian said, "It is either brown, or gray". Charlie said "I know it is brown". I said, "At least one of you is right and at least one of you is wrong." What is the color of my horse if the color is one of the above?

Allan says 'not black', Brian says 'brown or gray', Charlie says 'brown'. At least one is right and at least one is wrong. If it's brown, Allan (not black) and Charlie (brown) would both be right, but only Brian would be wrong - this satisfies 'at least one right, one wrong'. However, if it's gray, Allan (not black) is right, Brian (brown or gray) is right, Charlie (brown) is wrong. This also satisfies the condition. But if it's black, Allan would be wrong, Charlie would be wrong, and only Brian would be wrong - giving 0 correct, violating 'at least one right'. So gray is the correct answer, which matches option C.