Which of the following scenarios would indicate a closed universe model?

A closed universe model is characterized by a finite, curved space that leads to eventual gravitational collapse. When we consider the implications of a closed universe, it suggests that the density of the universe is higher than a critical threshold, resulting in the gravitational pull of its matter overcoming the expansion caused by the Big Bang. This scenario means that after a period of expansion, the universe will eventually come back together, leading to a "Big Crunch."

The other scenarios represent different models of the universe. For instance, an expansion forever without deceleration relates to an open universe model, where the universe expands indefinitely. An expansion with no observable boundary describes a flat universe, where the geometry is such that it could expand infinitely without curving back on itself. Lastly, a universe that reaches a steady state implies a balance where expansion continues but does not lead to collapse or infinite growth, aligning more closely with the steady state theory that was once popular but has largely fallen out of favor with current observations.

Therefore, the scenario of a universe that eventually collapses on itself succinctly captures the essence of a closed universe model.