**Explanation:**

**Plants:**
Plants are a primary source of food for many living organisms, including humans and animals. They produce their own food through a process called photosynthesis, where they convert sunlight, water, and carbon dioxide into glucose (a type of sugar) and oxygen. Plants provide a wide variety of foods such as fruits, vegetables, grains, nuts, and seeds. These foods are rich in essential nutrients like carbohydrates, proteins, vitamins, and minerals, which are necessary for the growth and development of living organisms.

**Animals:**
Animals are another source of food for humans and other animals. They obtain their food by consuming plants or other animals. Herbivores primarily feed on plants, while carnivores eat other animals. Omnivores have a diet that includes both plant and animal matter. Animals provide essential nutrients, like proteins and fats, which are important for the functioning of the body. Examples of animal-based food sources include meat, fish, poultry, eggs, dairy products, and honey.

**Humans:**
While humans are not typically considered a source of food, in extreme situations such as survival scenarios or cases of cannibalism, humans can indeed be a source of food for other humans. However, it is important to note that consuming human flesh is highly unethical and illegal in most societies.

**Both [a] and [b]:**
The correct answer is option 'D' because both plants (option 'a') and animals (option 'b') are sources of food. Humans rely on both plants and animals for sustenance, obtaining essential nutrients and energy from the consumption of various plant-based and animal-based foods. A balanced diet that includes a variety of plants and animals is crucial for maintaining good health and nutrition.