Final answer:
The average annual temperature on climate graphs is significant because it summarizes the annual temperature trends and reflects long-term average weather patterns that affect biomes and agricultural productivity.
Step-by-step explanation:
The average annual temperature is significant in climate graphs because it provides a summary of annual temperature trends, which is option C. It reflects the long-term average weather patterns, helping to identify climatic types and indicating potential agricultural productivity and biomes. Unlike daily temperature fluctuations or extremes, the average annual temperature gives a broader perspective on the climate of a region. This average is essential for understanding climate change, as even small changes in average temperature can have significant impacts on the global environment.