A. an example of a sign of a pathogen on a diseased plant is a. soft rot b. a canker on a plant stem c. stunted growth d. mycelium of a pathogen on the plant surface.

b. a canker on a plant stem

Explanation:

A canker on a plant stem -

It is the clear and strong evidence of a plant suffering from a disease which is due to pathogens.

According to the question ,  other options ,  stunted growth and soft rot, are not caused because of  pathogens , they are due to genetic reasons .

And, the option, mycelium of a pathogen , is not a plant disease .

Hence , incorrect option .

Therefore , from the options , the correct one is - a canker on a plant stem .