What surrounds the nucleus of a cell as a double membrane?

The structure that surrounds the nucleus of a cell as a double membrane is known as the nuclear envelope. This envelope is composed of two lipid bilayers—the inner and outer membranes—which enclose the nucleoplasm and form a barrier between the nucleus and the cytoplasm. This double membrane structure is crucial because it not only protects the genetic material within the nucleus but also regulates the passage of ions, RNA, and proteins in and out of the nucleus through nuclear pores. The nuclear envelope plays a vital role in maintaining the integrity of the cell’s genetic information and facilitating communication between the nucleus and the cytoplasm.

In contrast, the other options refer to membranes that are involved in enclosing the entire cell (like the cell membrane and plasma membrane) or structures within the cytoplasm (like the cytoplasmic membrane), which are not specific to the nucleus.