Which part of the cell maintains homeostasis of the cell?

The correct option is A Cell membrane
The cell wall is the outermost structure of plant cells. But it is not involved in checking the particles entering or leaving the cell. It confers the protection from wear and tear, pathogen attack, etc. Therefore, it is not involved in the homeostasis of the cell.
Plastids are present in plant cells, which synthesize and store important chemical compounds used by the cell.
Mitochondria produce the energy currency, i.e., ATPs of the cell. Mitochondria take nutrients and convert them into energy molecules. This process is known as 'oxidative phosphorylation'.

The cell membrane is the outermost covering of most cells. It is a semipermeable membrane and checks the molecules entering and leaving the cell by different mechanisms. It takes useful particles inside the cell by forming invaginations around them and taking them inside by the process of endocytosis. It also throws cellular waste by exocytosis. It allows the passing of useful particles inside the cell through various transport molecules and structures. It does not allow entry for harmful particles. Thus, the correct answer is option A.