**Core Concept**
The question is testing the student's knowledge of the anatomy and physiology of the extrinsic muscles of the jaw, specifically the cranial nerves that control these muscles. The tetanus infection has caused spasms and rigidity of the jaw muscles, leading to the inability to open the mouth.

**Why the Correct Answer is Right**
The correct answer is the muscle that is responsible for opening the mouth, which is the lateral pterygoid muscle. This muscle is innervated by the mandibular division of the trigeminal nerve (cranial nerve V3). The tetanus infection has caused spasms and rigidity of the jaw muscles, leading to the inability to open the mouth. The lateral pterygoid muscle plays a crucial role in opening the jaw by pulling the condyle of the mandible forward and downward. The spasms of the lateral pterygoid muscle due to tetanus have resulted in the inability to open the jaw.

**Why Each Wrong Option is Incorrect**
**Option A:** Masseter muscle - This muscle is responsible for elevating the mandible and is also innervated by the mandibular division of the trigeminal nerve. While it is involved in the movement of the jaw, it is not the primary muscle responsible for opening the mouth.

**Option B:** Medial pterygoid muscle - This muscle is also responsible for elevating the mandible and is innervated by the mandibular division of the trigeminal nerve. It is not the primary muscle responsible for opening the mouth.

**Option D:** Temporalis muscle - This muscle is responsible for elevating the mandible and is innervated by the mandibular division of the trigeminal nerve. It is not the primary muscle responsible for opening the mouth.

**Clinical Pearl / High-Yield Fact**
Tetanus infection can cause spasms and rigidity of the jaw muscles, leading to the inability to open the mouth. This is due to the tetanus toxin's effect on the inhibitory neurons of the spinal cord, leading to uncontrolled muscle spasms.

**Correct Answer:** C.