Is 15 grams one carbohydrate

15 =/= 1, you're going to need to give this question more context.

If you're into subtractive carb counting, you can take total carbs and subtract fiber for 'net' carbs. The theory is that fiber doesn't 'count' as a carb because the body can not break it down.

15 total - 14 fiber = 1 net carb