What might indicate that a tree is in poor health?

A tree's health can often be assessed by its observable characteristics, and persistent leaf drop alongside discoloration is a significant indicator of trouble. When a tree consistently sheds its leaves or shows a lack of vibrancy in color, it suggests that the tree might be under stress due to various factors. This could include inadequate water supply, nutrient deficiencies, disease, or pest infestations. Such symptoms indicate that the tree is struggling to maintain its health and vitality, which makes option C a clear sign of a tree in poor health.

Other options suggest positive health indicators. Vibrant leaf color and rapid growth are generally associated with a well-nourished and healthy tree, while strong and healthy bark signifies protection against pests and environmental stressors. These features are contrary to signs of poor health, which is why they would not be correct indicators in this context.