Final answer:
The correct answer to the question is (4) facilitated diffusion, where substances move across a plasma membrane with the help of transport proteins down a concentration gradient, without cellular energy expenditure.
Step-by-step explanation:
Diffusion during which the substance to be transported binds selectively to a membrane-spanning protein, which helps the process along, is called facilitated diffusion. This process involves material moving across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient, without the expenditure of cellular energy. Facilitated diffusion is essential for the transport of polar substances and other solutes that do not diffuse easily or quickly across the plasma membrane due to its hydrophobic nature.
Transport proteins, such as channel proteins and carrier proteins, play a crucial role in facilitated diffusion. They form channels or pores that allow certain substances to pass through the membrane, or they act as carriers that bind to a substance and help it to move across the membrane.
The correct answer to the question is option (4) facilitated diffusion, which is a type of passive transport not requiring energy from the cell.