An outer membrane is not present in

The correct option is "nucleoli".

To understand why this is the correct answer, let's look at the membrane structure of the organelles listed in the options:
1. Lysosomes: These are membrane-bound organelles containing digestive enzymes. They are surrounded by a single outer membrane.
2. Plastids: Organelles such as chloroplasts are double-membrane-bound, meaning they possess both an inner membrane and an outer membrane.
3. Mitochondria: Like plastids, mitochondria are double-membraned organelles, consisting of an outer membrane and a folded inner membrane (cristae).
4. Nucleoli: The nucleolus (plural: nucleoli) is a dense sub-nuclear structure located inside the nucleus where ribosome biogenesis occurs. Unlike organelles, the nucleolus is a non-membrane-bound macromolecular assembly. Since it lacks any surrounding membrane, an outer membrane is not present in nucleoli.