Which is the most common agent employed in external bleaching?
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
Sodium perborate
Hydrochloric acid
Carbamide peroxide
Sodium hypochlorite