Hint: The layer found in all cells which separate the cell's inside from the outside world. In bacterial and plant cells, an exterior surface of a cell wall is connected to this layer.
> Option A is correct. The plasma membrane is a biological membrane that separates the internality of all cells from the external environment (the extracellular space) that protects the cell from its environment. The plasma membrane of all cells represents the universal boundary between the cell and its surroundings. It is the outermost insulation of the animal cells as it is elastic and the property of semipermeable membrane helps control what enters and exits the cell.
> Option B is incorrect. The nucleus is a membranous-bound organelle comprising eukaryotic organisms' genetic material (DNA). As such it helps to preserve the cell's integrity by promoting processes of transcription and replication. It's the largest organelle inside the cell that makes up about a tenth of the total volume of the cell.
> Option C is incorrect. The cytoplasm is a dense solvent that fills each cell and is bound by the membrane of the cells. It's made mainly of water, salts, and proteins. The cytoplasm in eukaryotic cells contains all of the material inside the cell and outside the nucleus. It provides the organelles with a means of staying suspended. It serves as a buffer and protects against damage caused by movement and collision with other cells, the genetic material of the cell, and also the cell organelles.
> Option D is incorrect. The cytoskeleton is a channel of filaments or tubules that is scattered across the cell via cytoplasm, which is the entire material within a cell except for the nucleus. Although the proteins it is made of differ from organism to organism, they are present in all cells. It allows plants to maintain their form and internal organization, as well as providing mechanical support that allows cells to perform essential functions such as division and movement.
Hence, The correct answer is option (A).
Note: A cell is protected by the plasma membrane or the cell membrane. It also provides a fixed surrounding within the cell. And there are several different functions to that membrane. One is to carry nutrients into the cell and to carry toxic substances out of the cell.