Christian paid $18 for the haircut. To calculate the tip, multiply the cost of the haircut by the tip percentage: $18 * 0.20 = $<<18*0.20=3.60>>3.60. Add the tip amount to the cost of the haircut to find the total amount paid: $18 + $3.60 = $<<18+3.60=21.60>>21.60. Christian paid $21.60 for the haircut. Answer: \boxed{C}.
