Answer:
b. a canker on a plant stem
Step-by-step 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 .