Induction is an essential extrinsic process in embryonic development where one group of cells influences the developmental fate of another based on their relative positions. It often involves signaling molecules—commonly proteins—secreted by inducing cells that bind to receptors on responding cells. This interaction triggers signaling cascades guiding proper tissue differentiation and pattern formation.

Statement A is correct: The inductive signal can be a protein secreted by an inducing cell that binds receptors on a responding cell, facilitating intracellular pathways that determine cell fate.

Statement B is incorrect because the response to inductive signals depends on the competence of the responding cell, not the inducing cell. Competence is the ability of the recipient cell to respond to inductive signals.

Statement C is incorrect. Instructive induction occurs when an inducing cell provides signals needed to specify or alter the fate of an uncommitted or unspecified responding cell. It does not happen when the responding cell is already committed.

Statement D is correct. Lateral inhibition is a form of inductive interaction where cells in a field inhibit their neighbors from adopting the same fate, resulting in the emergence of evenly spaced differentiated cells, creating patterned tissue.

Thus, the correct combination is:

(3) A and D