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 answer:
Explanation of the incorrect options:
Option B: Microorganisms that cause many diseases are called pathogens.
Option C: An infected person is called a carrier of the disease.
Option D: Diseased plants are infested by pests and are causes for disease transmission, not agents of transfer or vector.
Final answer: Vectors can be defined as animals carrying the infecting agents from sick person to another healthy person.