Where are you most likely to find plant roots

The correct answer is A horizon.  

A horizon is also called topsoil and is situated just beneath the O horizon. This layer comprises of decomposed organic matter and minerals and is also very dark in color, known as humus. The plant roots grow mainly in this layer. The humus comprises of nutrients, which feed plants.  

The humus also soaks up and holds water more easily. The majority of the plant roots grow in this soil horizon and the plants absorb nutrients and water from this layer.