What type of curve is used to illustrate the increase in revenue or productivity over time?

The correct choice for illustrating the increase in revenue or productivity over time is the exponential curve. An exponential curve effectively represents scenarios where growth accelerates over time, which is common in fields like finance, biology, and technology. This type of growth indicates that as time progresses, the rate of increase becomes more significant, leading to rapidly compounding results.

Exponential growth can illustrate phenomena such as the adoption of new technologies, market expansion, or cumulative revenue generation, all of which typically do not increase at a constant rate. Instead, they often start slowly but pick up speed as more investment or momentum builds.

Other choices present different types of growth modeling: a linear curve depicts constant growth over time; a diffusion curve illustrates how new innovations are adopted within a market; and a parabolic curve often indicates scenarios where there are diminishing returns after reaching a peak. Each of these curves represents unique trends that are not suitable for representing the accelerated growth of revenue or productivity seen with exponential growth.