The law _______ farmers not only to save and resow (multiply) seeds, but also to sell them to other farmers, no matter what the original source of the seed is.

Here, “permits” which means “officially allow (someone) to do something” perfectly fits in the given blank making the sentence grammatically correct and contextually meaningful. Hence, option 4 is the most suitable answer choice.

Coverts: a thicket in which game can hide.

Implements: put (a decision, plan, agreement, etc.) into effect.

Emits: produce and discharge (something, especially gas or radiation).

Deserts: what a person deserves with regard to reward or (more usually) punishment.