Answer:
The statement (a) correctly describes that why cells are so small.
Step-by-step explanation:
Cells are formed of cytoplasm enveloped by a phospholipid bilayer. In the cells of plants, a supplementary cell wall also prevails. The cytoplasm accommodates various metabolic activities that need a spontaneous supply of nutrients. During the same time, metabolic wastes originate from these reactions, which are required to be eradicated. All this uptake and discard of nutrients and waste components occurs by mediation across the cell membrane via passive, active, or facilitated transport.
In the absence of any circulatory system, these cells depend entirely on the process of diffusion through the membrane. The brisk interchange of molecules can only take place if diffusion occurs over brief distances. Thus, this rate of uptake of nutrients within the cells and discarding of wastes into the external surroundings hugely relies upon the cell surface, which is restricted by the cell membrane. Thus, surface area of the cell is one of the limiting factors to monitor the rates of diffusion of these molecules. Therefore, when the cells are small, their wastes and food can be handled effectively by the cell membrane.