**Core Concept**
The question tests knowledge of cellular structures, specifically those with or without membranes. In cell biology, **membranes** are crucial for enclosing and protecting various organelles, but some structures lack them.

**Why the Correct Answer is Right**
Given the blank options, a likely correct answer could be related to **ribosomes**, which are involved in protein synthesis and do not have a membrane enclosing them. They are found throughout the cytoplasm and are not membrane-bound, unlike other organelles such as the **nucleus**, **mitochondria**, or **endoplasmic reticulum**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific text, we can't directly address this option, but typically, structures like the **nucleus** are enclosed by a double membrane.
**Option B:** Similarly, without text, but structures like **mitochondria** are known for their double membrane structure.
**Option D:** Again, lacking text, but structures such as **lysosomes** are membrane-bound, containing digestive enzymes.

**Clinical Pearl / High-Yield Fact**
Remember, **ribosomes** are key for protein synthesis and are unique because they are not enclosed by a membrane, making them distinct from other organelles in the cell.

**Correct Answer:** D. Ribosomes.