These are considered bone - dissolving cells.

Options

A. Osteoprogenitor cells
B. Osteoclasts
C. Oscteocytes
D. Osteoblasts

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the type of cells that are known as "bone-dissolving cells." This indicates that we are looking for a specific type of cell involved in the process of bone resorption.

2. **Analyzing the Options**:
- **Option 1: Osteoprogenitor Cells**: These are stem cells that give rise to osteoblasts, which are involved in bone formation. Therefore, this option is incorrect as they do not dissolve bone.
- **Option 2: Osteoclasts**: These cells are responsible for the resorption of bone tissue. The term "osteoclast" comes from the Greek words "osteo," meaning bone, and "clast," meaning to break. This option is correct as osteoclasts are indeed the bone-dissolving cells.
- **Option 3: Osteocytes**: These are mature bone cells that maintain bone tissue. They are located within the bone matrix and do not dissolve bone. Thus, this option is incorrect.
- **Option 4: Osteoblasts**: These cells are responsible for bone formation. They build new bone and are not involved in the dissolution of bone. Therefore, this option is also incorrect.

3. **Conclusion**: After reviewing all the options, the correct answer is **Option 2: Osteoclasts**, as they are the cells that dissolve bone.