Biology

The correct answer is: A.Ligands bond to receptor proteins and cause the protein to relay a message to intracellular proteins.

Explanation:

Ligands are signaling molecules that bind to specific receptors on the cell surface or inside the cell. Once bound, the receptor typically undergoes a conformational change that activates intracellular signaling pathways, leading to a cellular response.

Here's why the other options are incorrect:

B.Ligands can bind to cell-surface receptors, but they cannot enter the cell: This is not universally true. While some ligands, especially hydrophilic ones, bind to receptors on the cell surface, ...