Concept:The shrinking of a spirogyra cell in a strong solution is caused by water loss from the cell.Explanation:A strong solution has a lower concentration of water compared to the cell sap inside the spirogyra.Due to this difference, water moves out of the cell by osmosis.As water leaves, the cytoplasm and cell membrane pull away from the rigid cell wall.This specific process of protoplasm shrinkage in a hypertonic solution is called plasmolysis.Answer:C. plasmolysis