Final Answer:
The model of the universe that gives the youngest age for its present size is an accelerating universe. An accelerating universe implies ongoing expansion at an increasing rate. The answer is D) an accelerating universe.
Step-by-step explanation:
The model of the universe that gives the youngest age for its present size is option D) an accelerating universe. In an accelerating universe, the expansion of the universe is not only ongoing but is also accelerating over time.
This acceleration results in a universe that is younger than what would be predicted based on a constant or decelerating expansion. Models A) recollapsing universe and B) coasting universe suggest deceleration in the expansion, leading to older age estimates. Model C) a critical universe represents the dividing line between acceleration and deceleration, but an accelerating universe is consistent with current observational evidence, providing a younger age for its present size.