An exceptional demand curve is one that slopes upward to the right. This means that as the price of a good or service increases, the quantity demanded also increases. This type of demand curve is uncommon because it goes against the general pattern of a downward-sloping demand curve, where as price increases, quantity demanded decreases.

Option 1, which states that the demand curve slopes downward to the right, is incorrect because this describes a regular demand curve. Option 3, which states that the demand curve slopes horizontally, is also incorrect because this represents a perfectly elastic demand, where quantity demanded remains the same regardless of price. Option 4, which states that the demand curve slopes upward to the left, is incorrect because this represents a negatively sloped demand curve, which is also not exceptional.

Therefore, the correct answer is option 2, which correctly describes an exceptional demand curve that slopes upward to the right.