Solution:
Concept: Heart spoilage of marigold is produced by the lack of Boron. Heart spoilage of marigold can be averted by utilising a small cure of sodium tetraborate in the gault. Thus, this is the correct answer. A small amount of sodium tetraborate can prevent heart rot disease in marigolds. For cell wall formation, boron plays a significant role. It is also useful for moving sugar to the growing part of the plants. The deficiency of Boron can affect the growth and development of the plant.Hence, option 3 is the correct answer.
