Which of the following best describes a visible sign of a plant disease?

The visible sign of a plant disease is often characterized by specific symptoms that indicate the presence of a pathogen. A bacterial ooze on stems is a clear and direct sign of a bacterial infection. This ooze is a result of the bacterial pathogens disrupting the plant's cellular structure, leading to the exudation of a substance that can be easily observed. It not only indicates the presence of the bacterial disease but also provides insight into its severity and impact on the plant's health.

In contrast, while the other options depict signs of plant distress, they are symptoms that may arise from a variety of stressors, not exclusively from disease. Stunted growth, leaf spots, and yellowing of leaves all represent nonspecific responses that can result from environmental factors, nutrient deficiencies, or various types of infections, making them less definitive as clear indicators of a particular plant disease. Hence, the presence of a bacterial ooze is the most unmistakable visible sign of a disease caused specifically by bacteria.