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.