Which of the following structures regulates the entrance and exist of molecules into and out of the cytoplasm ?

The structure that regulates the entrance and exit of molecules into and out of the cytoplasm is the cell membrane (plasma membrane). 

The cell membrane is a selectively permeable barrier that separates the cell's internal environment (cytoplasm) from the extracellular environment.

The cell membrane is composed primarily of a lipid bilayer with embedded proteins. These proteins play a crucial role in regulating the passage of molecules.