Explanation:

Cotyledons: Cotyledons are embryonic leaves that are present within the seed of a flowering plant. They are also known as seed leaves as they emerge from the seed upon germination and are the first leaves to appear on a seedling. Cotyledons provide the necessary nutrients for the growth of the young seedling until it can produce its own leaves and begin photosynthesis. The number of cotyledons varies among plant species. Some plants have one cotyledon, while others have two or more.

Seedling: A seedling is a young plant that has just started to grow from a seed. It is usually characterized by the presence of cotyledons, which are the first leaves to appear. As the seedling grows, it develops true leaves that are different in shape and size from the cotyledons. The true leaves are capable of photosynthesis and take over the role of producing food for the plant.

Seed coat: The seed coat is the outer layer of the seed that protects the embryo inside. It is usually hard and durable to protect the seed from damage and ensure proper germination. The seed coat also helps to regulate the exchange of gases and water between the seed and the environment.

Stem: The stem is the main structural axis of a plant that supports the leaves, flowers, and fruits. It also transports water, minerals, and nutrients from the roots to the rest of the plant. The stem is usually green and photosynthetic in young plants, but becomes woody and brown in mature plants.

Therefore, the correct answer is option B, cotyledons are also known as seed leaves.