The standard and humane sequence of operations in a poultry slaughtering unit is as follows:
A. Stunning: The bird is first rendered unconscious.
C. Bleeding (Exsanguination): The bird is bled while it is unconscious.
B. Scalding: The carcass is dipped in hot water to loosen the feathers.
D. Evisceration: The internal organs are removed from the carcass.
Therefore, the correct sequence is A, C, B, D.