The correct answer is (a) by use
Earlier the people were not aware about the genetics and classification of plants and animals. So, they used to classify plants and animals by their use. The plants were used for many purposes such as eating, medicine, cosmetics. Either the plants were usable or non usable. Plants like neem, tulsi, ginger, turmeric were used as medicine. Animals were used for obtained dairy products like milk and butter, for eating, for exacting colors out of them.