**Explanation:**

The correct answer is option **B) RBC (Red Blood Cell)**.

The lack of a nucleus is a characteristic feature of RBCs. Here's a detailed explanation of why RBCs lack a nucleus:

**1. Structure of RBCs:**
Red blood cells, also known as erythrocytes, are the most abundant type of blood cells in the human body. They are responsible for carrying oxygen from the lungs to the rest of the body and removing carbon dioxide. RBCs have a unique structure that allows them to perform their function effectively. They are small, disc-shaped cells with a biconcave shape.

**2. Development of RBCs:**
RBCs are produced in the bone marrow through a process called erythropoiesis. During this process, precursor cells called erythroblasts differentiate and mature into RBCs. As the RBCs mature, they shed their nucleus, leaving them without one. This allows them to have more space to carry oxygen and increases their flexibility, enabling them to squeeze through narrow capillaries.

**3. Function of RBCs:**
The lack of a nucleus in RBCs allows for more space to accommodate hemoglobin, the protein responsible for binding and transporting oxygen. The absence of a nucleus also increases the flexibility of RBCs, enabling them to deform and squeeze through the narrowest capillaries, ensuring efficient oxygen delivery to tissues. Additionally, the lack of a nucleus means that RBCs have a limited lifespan and are constantly replaced by new cells.

**4. Comparison with other cell types:**
a) **WBC (White Blood Cell):** Unlike RBCs, WBCs do have a nucleus. White blood cells are a crucial part of the immune system and are responsible for fighting off infections and foreign substances. Their nucleus contains genetic material and other components necessary for their function.

b) **Nerve Cell:** Nerve cells, also known as neurons, are specialized cells responsible for transmitting electrical signals in the nervous system. Neurons have a distinct shape with long extensions called dendrites and axons. They have a nucleus that contains genetic material and other components essential for their function.

c) **Muscle Cell:** Muscle cells, also known as muscle fibers, are long and cylindrical cells responsible for contraction and movement. They have a nucleus that contains genetic material and other components necessary for muscle function.

Therefore, among the given options, the cell type that lacks a nucleus is RBC (option B).