The correct answer is: c. through a conformational change of the receptor
When a ligand binds to receptor that is located on a cell surface, signal has to be transported into the cell. Usually, intracellular domain of the receptor changes its “shape” (conformation) which may make it active as an enzyme or available for other molecules binding. For example, the receptor may turn on signaling molecule inside of the cell and activates further chain reactions. Consequently, the cell responses by changing its behavior or characteristics.