What is a good emergency carbohydrate snack for when your blood sugar is low?

15g simple carbs. Assuming you mean for a diabetic that can eat, 15 grams of simple carbs (things that taste sweet, made of real sugar) such as glucose tablets, juice, quick dissolving candy. Complex carbs that require digestion (bread, pasta) will take longer to raise blood sugar. Avoid snacks with fat, bc the fat will slow absorption too.
Simple sugar. The key is to eat/drink something that can be absorbed quickly. Some 15 gram sugar ideas would be 3 glucose tablets, 5-6 lifesavers*, 1/2 can of soda*, or 4-6oz orange juice. (*remember not to use sugar-free or diet candy or sodas) avoid foods with fat or protein that will take longer to digest. If you won't be eating a meal soon, eat a snack like peanut butter crackers or a half sandwich.