Final answer:
The evolutionary advantage of Spirogyra being multicellular is increased efficiency in reproduction and survival.
Step-by-step explanation:
The evolutionary advantage of Spirogyra being multicellular is that it allows for increased efficiency in reproduction and survival. Multicellularity allows Spirogyra to have specialized cells for different functions, such as reproductive cells and cells for photosynthesis. This division of labor improves the overall functioning of the organism and increases its chances of survival.