Which is the most common agent employed in external bleaching?: (A) Sodium perborate, (B) Hydrochloric acid, (C) Carbamide peroxide, (D) Sodium hypochlorite
Answer is C. The most common bleaching agent for at-home bleaching is 10% to 22% carbamide peroxide with an effective yield of 4% to 7.5% hydrogen peroxide.
Ref: Cohen pathways of Pulp 11th ed page no  e107