Vectors can be defined as:

The correct option is A

animals carrying the infecting agents from sick person to another healthy person

The correct option is A.

Explanation of the correct answer:

1. Animals that carry the infecting agents from an infected individual to another healthy individual.
2. Vector-borne diseases are illnesses spread through vectors.
3. A vector is a carrier of the causative microbe for various diseases such as mosquitoes, ticks, and fleas.
4. A vector is an organism (known as arthropod vectors) that carries and transmits any infectious pathogen from an amplifying host to other living organisms.
5. These vectors may be parasites or other microbes.
6. The diseases spread by vectors are: Dengue, Malaria, Giardia, Leishmaniasis, Trypanosomiasis, Filariasis, etc.

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.