In diagnosing American Foulbrood, which of the following signs would be observed?

When diagnosing American Foulbrood (AFB), the sign of ropiness when agitated is a key indicator of the disease. This characteristic occurs due to the presence of the bacteria, Paenibacillus larvae, which infects the larvae of honeybees. When the infected larva is disturbed or agitated, it releases a sticky, ropy, and stringy substance that can stretch and cling together. This ropiness is a defining characteristic of AFB and is not commonly seen with other diseases, making it a reliable diagnostic feature.

The other signs listed may indicate other issues or conditions. Hardened larvae, for instance, can be associated with other diseases or environmental stress rather than AFB alone. Similarly, dark-colored dead larvae may result from various factors, including other infections or even natural processes. The presence of pupal tongues in empty cells is characteristic of certain brood diseases but does not specifically point to AFB. Thus, the unique diagnostic sign of ropiness when agitated makes it the correct answer for identifying American Foulbrood.