Choose two correctly paired sets
(a) Sphenoid bone-Keystone-Cranium (b) Acromion process-collar bone-scapula (c ) Xiphoid process-breast bone-thorax (d) Odontoid process-atlas-cervical vertebra

Options

A. a,d
B. b,d
C. b,c
D. a,c.

Answer

To solve the question of choosing two correctly paired sets from the given options, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option (a): Sphenoid bone - Keystone - Cranium**
- The sphenoid bone is often referred to as the "keystone" of the cranial floor because it articulates with all other cranial bones.
- **Conclusion**: This pairing is correct.

2. **Option (b): Acromion process - Collar bone - Scapula**
- The acromion process is a bony process on the scapula (shoulder blade). However, the term "collar bone" refers to the clavicle, which is not the same as the scapula.
- **Conclusion**: This pairing is incorrect.

3. **Option (c): Xiphoid process - Breast bone - Thorax**
- The xiphoid process is indeed a part of the sternum (breastbone) and is located in the thoracic region.
- **Conclusion**: This pairing is correct.

4. **Option (d): Odontoid process - Atlas - Cervical vertebra**
- The odontoid process (or dens) is a bony projection of the second cervical vertebra (axis) and not the first (atlas). Therefore, this pairing is incorrect.
- **Conclusion**: This pairing is incorrect.

### Final Answer:
The two correctly paired sets are:
- (a) Sphenoid bone - Keystone - Cranium
- (c) Xiphoid process - Breast bone - Thorax

Thus, the correct answer is option (a) and (c).