Final answer:
The upper limit of cell size is determined by the surface area-to-volume ratio. As a cell increases in size, its surface area-to-volume ratio decreases. The correct answer is option a).
Step-by-step explanation:
As a cell increases in size, its surface area-to-volume ratio decreases. This is because the surface area of a cell increases as the square of its radius, while the volume increases as the cube of its radius. For example, if a cell doubles in size, its surface area will increase by a factor of four, while its volume will increase by a factor of eight.
Therefore, if a cell grows too large, the plasma membrane will not have enough surface area to support the rate of diffusion required for the increased volume. As a result, the upper limit of cell size is determined by the surface area-to-volume ratio.
While the surface area-to-volume ratio is the primary determinant of the upper limit of cell size, other factors such as nucleus size, number of organelles, and type of cell membrane can also affect cell size to some extent. However, these factors do not play as significant a role as the surface area-to-volume ratio.