What is the purpose of hardening? To eliminate all disease
To manage the population of endangered trees
To allow nursery plants to transition to the outdoors
To make sure that the trunk of the tree is as strong as possible before planting

To allow nursery plants to transition to the outdoors.

Explanation:

Hardening is a process in which the small seedling plants are exposed to the unprotected environment of the garden so that they develop some tolerance against those conditions. The small seedlings were earlier exposed to low water, sunlight, and wind conditions but for hardening they are kept at those areas of the garden where they experience harsh levels of these agents for a short duration and the duration is increased with routine so that plants can adapt the changes when they are planted outdoor or outside the care of garden. It is one of the gardening practices.  

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