Answer:
The plant roots are most likely found at "horizon".
Step-by-step explanation:
Plant roots are extremely likely to occur in topsoil (horizon). Plant roots are organs which consume water and nutrients. Topsoil or front layer consists of minerals and decomposed organic matter and is also very dense in color, known as humus.
In this layer the plant roots grow primarily. The humus is composed of nutrients that sustain plants. The humus often soaks up and is easier to hold water. In this soil horizon the plurality of plant roots expand, and plants consume nutrients and water from this coating.