Explanation:

- The word "prolong" means to extend the duration of something or to make it last longer.

- Option 1: Allow

- "Allow" means to let something happen or permit, which does not directly relate to time or length.

- Option 2: Shorten

- "Shorten" means to make something shorter or reduce its length in time.

- This is directly opposite in meaning to "prolong" and is the most accurate antonym.

- Correct Answer

- Option 3: Increase

- "Increase" refers to making something larger or more in quantity, not length of duration.

- Option 4: Prevent

- "Prevent" means to stop something from happening, not specifically related to extending or reducing duration.