Which of the following is not a characteristic of red giants?

The choice of "high temperature" as the answer is insightful because red giants are typically characterized by their low surface temperatures despite their high luminosity. As stars evolve into the red giant stage, they exhaust the hydrogen in their cores and begin to fuse helium, which causes their outer layers to expand significantly. This expansion leads to a larger size and results in lower temperatures for their outer shells, contributing to their reddish appearance.

In contrast, red giants possess high luminosity due to the vast amount of energy being produced by nuclear fusion in their expanded cores, and they are indeed very large as their outer layers have expanded significantly. Additionally, they represent a late-stage evolution in a star's life cycle, specifically after the main sequence phase. Hence, their defining characteristics do not include high surface temperatures, solidifying why this option stands out as not true for red giants.