The correct answer is Option 3: Alginate.

- Alginate is an irreversible hydrocolloid impression material widely used in dentistry due to its ease of manipulation and cost-effectiveness.
- However, it is highly prone to dimensional changes if the cast is not poured promptly after taking the impression.
- This is primarily because alginate contains a significant amount of water, which can evaporate (syneresis) or be absorbed from the surrounding environment (imbibition).

- Syneresis: Loss of water leading to shrinkage of the impression.
- Imbibition: Absorption of water causing swelling of the impression.

- Due to these characteristics, alginate impressions must be poured within 10-15 minutes to ensure dimensional accuracy and prevent distortions in the final cast.

In contrast:
- Polyvinyl siloxane is a highly stable addition silicone impression material that exhibits excellent dimensional stability even if pouring is delayed for several days.
- Zinc oxide eugenol impression material is rigid and dimensionally stable but not flexible, hence it is less prone to dimensional changes with delayed pouring.
- Polysulfide rubber has good dimensional stability and can tolerate some delay before pouring.

Therefore, among the given options, alginate is the most prone to dimensional changes if the cast is not poured without delay.

Reference: "Sturdevant's Art and Science of Operative Dentistry," Volume 1, Chapter on Impression Materials, Page 216-220