The correct answer is option 'A', the placenta. The placenta plays a crucial role in providing nutrients to the growing fetus inside the uterus.

The placenta is an organ that develops during pregnancy and attaches itself to the wall of the uterus. It acts as a lifeline between the mother and the fetus, supplying necessary nutrients and oxygen to support the growth and development of the fetus.

Here is a detailed explanation of how the placenta provides nutrients to the fetus:

1. Formation of the placenta:
- After fertilization, the fertilized egg implants itself into the lining of the uterus.
- Specialized cells from the outer layer of the embryo and the lining of the uterus combine to form the placenta.
- The placenta is connected to the fetus by the umbilical cord, which contains blood vessels that carry oxygen and nutrients to the fetus and remove waste products.

2. Oxygen supply:
- The placenta receives oxygen from the mother's bloodstream through the walls of the uterus.
- Oxygen-rich blood from the mother enters the placenta through the maternal arteries.
- The oxygen diffuses across the placental membrane into the fetal blood vessels in the umbilical cord, which then carries it to the fetus.

3. Nutrient supply:
- The placenta also receives nutrients from the mother's bloodstream.
- Nutrient-rich blood from the mother enters the placenta through the maternal arteries.
- These nutrients, including glucose, amino acids, and fatty acids, pass through the placental membrane into the fetal blood vessels in the umbilical cord.
- The nutrients are then transported to the fetus, providing the necessary energy and building blocks for its growth and development.

4. Waste removal:
- As the fetus grows, waste products such as carbon dioxide and urea are produced.
- These waste products are removed from the fetal blood vessels in the umbilical cord and transported back to the placenta.
- The placenta then releases these waste products into the mother's bloodstream, where they are eliminated through the mother's excretory system.

In summary, the placenta is responsible for supplying oxygen and essential nutrients to the growing fetus inside the uterus. It acts as a conduit between the mother and the fetus, ensuring that the developing baby receives everything it needs for healthy growth and development.