Which of the following can be considered a definite indicator of decay?

Carpenter ants serve as a strong indicator of decay because they typically infest trees that are already compromised by internal decay. These ants prefer to create nests in wood that is softened by fungal decay, which means their presence suggests that the wood has lost structural integrity. Unlike other indicators, carpenter ants imply an active decomposition process, indicating that the tree is not merely experiencing surface damage or other types of stress but is undergoing significant internal deterioration.

Understanding this, other options may provide insights into tree health but lack the definitive nature of carpenter ants in indicating internal decay. For instance, dead or loose bark can indicate many issues, including mechanical damage or environmental stress, rather than specifically identifying decay. Similarly, sunken areas may suggest structural issues but do not directly confirm the presence of decay. Response growth patterns, while informative about how a tree is reacting to various stresses, do not adequately pinpoint decay on their own. Therefore, the presence of carpenter ants is the most telling sign of decay among the choices, making it the most reliable indicator in the context of tree risk assessment.