Final answer:
A signaling molecule is known as a ligand or a first messenger, which binds to a specific receptor on a cell surface. Second messengers, like cAMP, propagate signals within the cell after the initial ligand-receptor interaction.
Step-by-step explanation:
A signaling molecule is also called a ligand or a first messenger. Therefore, the correct answer is D. both A and B. A ligand is a molecule that binds to a specific receptor on a target cell, typically initiating a signaling pathway inside the cell. Second messengers are different; they are small molecules within the cell that propagate a signal after the initial ligand-receptor interaction has occurred, working to amplify and spread the signal to different parts of the cell. An example of a second messenger is cyclic AMP (cAMP).