Easy question. answer should include explanation.

Rice Plants need to stand in a lot of water for growth. The Borders (bunds) of a rice field have to be Strong to hold the Water. The Farmer Planted Grass Plants on the Borders because, Grass has Fibrous roots which bind the soil of the bund and hold it together.

Explanation : The Answer itself Explains us well about why the farmer planted those grass plants (i.e.) to increase the strength of the soil. We know that Unity and Teamwork are Strong Words. The Soil and Fibrous Roots show us How Strong those Words are!

b is the best answer for this question.