What is a common mechanism by which cell surface receptors are activated?

• A. Ligand binding directly phosphorylates the receptor's cytosolic domain.
• B. Ligand binding results in the receptor's dissociation from the cell membrane.
• C. Ligand binding causes the receptor to be internalized and degraded.
• D. Ligand binding induces a conformational change in the receptor, activating its signaling function.

Answer: (D)