What are the three principal parts of a cell?

The three principal parts of a cell are indeed the plasma membrane, nucleus, and cytoplasm. The plasma membrane is critical as it serves as the protective barrier that regulates what enters and exits the cell, maintaining homeostasis. The nucleus is often referred to as the control center of the cell because it houses the cell's genetic material (DNA) and is responsible for regulating gene expression and the replication of DNA during cell division. The cytoplasm is the jelly-like substance that fills the cell and is where the organelles are located, providing a medium for biochemical reactions to occur.

The emphasis on "plasma membrane" rather than "cell membrane" in the correct answer reflects a more specific terminology commonly used in biological sciences, although both terms can be interchangeably used in a general context. Other options do not fully encompass the essential components recognized in cell biology, especially the inclusion of the cell wall, which is typically found only in plant cells and some bacteria, thus not applicable to all cell types. Mitochondria, although important organelles responsible for energy production, do not qualify as one of the three fundamental components categorizing every cell.