Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
when adding organic materials to a compost pile, gardeners should avoid animal products and fats in favor of scraps from plants that have not been treated with pesticides.

Explanation:

First, identify the relative clause. The relative clause here is "that have not been treated with pesticides". It is a restrictive relative clause because it provides essential information to identify which plants' scraps are being referred to (those not treated with pesticides). Restrictive relative clauses are not set off with commas as they are necessary for the meaning of the sentence.

Answer:

that have not been treated with pesticides