The graph shows four possible relationships between ambient temperature and body temperature - HSC - SSCE Biology - Question 12 - 2018 - Paper 1

To identify which line represents the relationship for an endotherm, we must understand that endotherms maintain a relatively constant body temperature despite changes in ambient temperature. In the graph:

• Line W shows a constant body temperature irrespective of ambient temperature, which is typical of ectothermic organisms.
• Line X indicates a slight increase in body temperature as ambient temperature increases, but is not perfectly stable.
• Line Y seems to align the body temperature with varying ambient temperatures, indicating it does not hold a constant body temperature.
• Line Z reflects a flat line at lower temperatures and then rises, suggesting it lacks the homeostasis typical of endotherms.

Therefore, the correct answer is B. X, as it best approximates the ability of an endotherm to regulate body temperature in response to ambient temperature changes.