The correct answer is option 'D', which is the plasma membrane.

The plasma membrane, also known as the cell membrane, is a thin, flexible barrier that surrounds the protoplasm of animal cells. It separates the cell's internal environment, or cytoplasm, from the external environment. The plasma membrane is made up of a double layer of phospholipids, which are arranged in a bilayer with their hydrophobic tails facing inward and their hydrophilic heads facing outward.

The plasma membrane plays a crucial role in maintaining the integrity and functionality of the cell. It regulates the passage of substances in and out of the cell, allowing for the selective transport of nutrients, ions, and waste products. This process is known as selective permeability. The plasma membrane contains various proteins, such as transport proteins and receptor proteins, which facilitate these processes.

The plasma membrane also helps to maintain the cell's shape and structure. It provides support and protection to the cell by acting as a barrier against harmful substances and preventing the loss of essential molecules. Additionally, the plasma membrane is involved in cell signaling and communication. It contains receptors that can detect and respond to specific molecules, allowing the cell to receive and process signals from its environment.

In contrast to plant cells, animal cells do not have a cell wall, which is a rigid structure made of cellulose. The cell wall is found in plant cells and some other organisms, providing additional support and protection. However, animal cells possess other structures, such as the nuclear membrane, which surrounds and protects the nucleus, and the tonoplast, which is a membrane surrounding the vacuole in certain animal cells.

In conclusion, the plasma membrane is the barrier between the protoplasm and the external environment in animal cells. It regulates the movement of substances in and out of the cell, maintains the cell's shape and structure, and facilitates cell signaling and communication.