Skull bones are

Options

A. Replacing bones
B. Investing bones
C. Sesamoid bones
D. Irregular bones.

Answer

### Step-by-Step Solution

1. **Understand the Types of Bones**:
- There are different classifications of bones in the human body, including replacing bones, investing bones, sesamoid bones, and irregular bones.

2. **Identify Skull Bones**:
- The question asks specifically about skull bones. Skull bones are the bones that form the structure of the skull, which protects the brain.

3. **Define Replacing Bones**:
- Replacing bones, also known as endochondral bones, are those that replace cartilage during development. They are formed through a process where a cartilage model is gradually replaced by bone tissue.

4. **Examine the Surgical Context**:
- When a section of the skull (referred to as a "bone flap") is removed during surgery to access the brain, it is typically replaced with tiny plates and screws. This indicates that the skull bones are capable of being replaced or repaired, aligning with the definition of replacing bones.

5. **Eliminate Other Options**:
- **Investing Bones**: These are also known as dermal bones, which ossify directly without a cartilage precursor. This does not apply to skull bones.
- **Sesamoid Bones**: These are small bones embedded within tendons, typically found in joints. Skull bones do not fit this category.
- **Irregular Bones**: These bones have complex shapes and do not fit into other categories. Skull bones have a specific structure and are not classified as irregular.

6. **Conclusion**:
- Based on the definitions and the surgical context, skull bones are classified as replacing bones. Therefore, the correct answer is that skull bones are replacing bones.