Vectors can be defined as
animals carrying the infecting agents from sick person to another healthy person
The correct option is A.
Explanation of the correct option:
Explanation of the incorrect options:
Option B:
Microorganisms that cause diseases are known as pathogens.
Option C:
An infected person who can infect a healthy person with the disease is not known as a vector.
Option D:
Diseased plants are not known as a vector.
Hence, vectors can be defined as animals carrying the infecting agents from sick person to another healthy person.