A plant can be made disease-resistant by

The correct option is A

Breeding

The correct option is A.
Explanation of the correct option:

Plants can be disease resistant by breeding them with a plant that has a resistance gene naturally.
In general, wild plant types have disease resistance genes, therefore breeding key crops with their wild variations produces disease-resistant plants.

Explanation of the incorrect option:
Option B:
Hormones are secondary messengers that regulate different functions in the body.
Option C:
Colchicine is a drug that inhibits the polymerization of tubulin protein and causes mitotic arrest.
Option D:
Heat is a form of energy that is transferred between the system and its surroundings.

Final answer:

A plant can be made disease-resistant by breeding.