Based on the description you've provided, where the outputs are constantly increasing but the rate of growth is decreasing, the best function to model this kind of data would be a logarithmic function.
Logarithmic functions increase continuously but at a decreasing rate, which fits the criteria you mentioned. Linear functions would have a constant rate of growth, while cubic functions might exhibit relative extrema. Absolute value functions typically show a "V" shape and do not fit the scenario described.
So, the best choice is a logarithmic function.