Final Answer:
The nucleolus is a structure within the nucleus of a cell and is not surrounded by a separate membrane. It is a dense region involved in the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes. In contrast, the nucleus is surrounded by a nuclear membrane, and both mitochondria and peroxisomes have their own membranes. So the corrrect option is B. Nucleolus
Step-by-step explanation:
The nucleolus is a substructure located within the nucleus of a cell and is not enclosed by a distinct membrane. It plays a pivotal role in ribosome biogenesis, orchestrating the synthesis and assembly of ribosomal RNA (rRNA) with proteins. While it lacks a surrounding membrane, the nucleus itself, indicated in option A, is enclosed by a double membrane, separating its contents from the cytoplasm.
In contrast, mitochondria (C) and peroxisomes (D) are membrane-bound organelles with specific functions related to energy production and metabolic processes. The nucleolus, despite its membrane-less state, is integral to cellular function, contributing to the cell's capacity to synthesize the molecular machinery essential for protein synthesis and cellular activities.