The correct answer is D
Explanation

It may be helpful to number the ribbons 1, 2, 3, 4, 5, and 6. Ribbon 1 can be worn with ribbons 2, 3, 4, 5, and 6, giving 5 combinations. Ribbon 2 can be worn with ribbons 3, 4, 5, and 6, giving 4 more combinations. (Do not recount ribbon 2 with ribbon 1, since that was counted in the first step.) Similarly, ribbon 3 can be worn with ribbons 4, 5, and 6, ribbon 4 can be worn with ribbons 5 and 6, and ribbon 5 can be worn with ribbon 6. The total number of combinations is 5 + 4 + 3 + 2 + 1 = 15.

 

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