Which structure is surrounded by two phospholipid bilayers?

1. The nuclear envelope is a double-membrane structure forming the outermost portion of the nucleus. 2. Both its inner and its outer membrane are phospholipid bilayers. 3. The nucleoplasm is the semi-solid fluid inside the nucleus, so it sits within both of those bilayers. 4. A vesicle is bounded by a single membrane, and a ribosome has no membrane at all. 5. The nucleoplasm is the structure enclosed by two phospholipid bilayers. 6. Cytoplasm lies outside the nuclear envelope altogether, so no part of it sits within those two bilayers. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 4 "Cell Structure", section 4.3 Eukaryotic Cells_