Where are you most likely to find plant roots

(horizon) is the correct answer.

Explanation:

The roots of plants are most likely to found in the (topsoil horizon).

Topsoil contains decayed organic material and minerals. The function of roots is to absorb nutrients and water. Roots are mostly grown in the topsoil of the earth comprise of water and nutrients. The roots are the main part of the plant that lies beneath the soil surface.

"A Horizon" aka topsoil