The plot that correctly shows the evolution of the universe would be B. Plot B.
Why does the plot show the evolution of the universe ?
When the mass density is less than the critical mass density, it means that the universe's actual density is below the density required for gravitational forces to halt the cosmic expansion.
In such a scenario, the universe would continue to expand at a constant rate due to the initial momentum from the Big Bang and the ongoing influence of gravity, which slows down the expansion but does not stop it completely.
This expansion at a constant rate is depicted as a steady/constant incline in a plot of the universe's evolution over time.
So, Plot B is the correct choice to represent the scenario where gravity is the only force in play.