**Core Concept**
The patient's symptoms suggest an issue with extraocular muscle function, specifically involving cranial nerves III, IV, and VI, which control eye movements. This condition is characterized by impaired extraocular muscle function, leading to restricted eye movements, ptosis, and absence of squint or diplopia.

**Why the Correct Answer is Right**
The diagnosis is likely due to a condition called **Myasthenia Gravis** (MG). MG is an autoimmune disorder where antibodies target the acetylcholine receptors at the neuromuscular junction, disrupting neurotransmission. This leads to muscle weakness and fatigue, especially affecting the extraocular muscles. The absence of diplopia (double vision) suggests that the disorder primarily affects the extraocular muscles, rather than the ocular motor nerves themselves.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically address the restricted eye movements and ptosis without squint or diplopia. While other conditions like thyroid ophthalmopathy can cause similar symptoms, they typically involve additional features like exophthalmos or lid retraction.

**Option B:** This option is incorrect because it refers to a condition that primarily affects the third cranial nerve (oculomotor nerve), which is not the likely cause of the patient's symptoms. The third nerve palsy typically presents with ptosis, but also involves pupillary involvement and often results in diplopia due to impaired lateral rectus function.

**Option C:** This option is incorrect because it does not accurately describe the patient's symptoms. While a cranial nerve III palsy can cause ptosis, it typically results in diplopia due to impaired lateral rectus function. Additionally, the pupil would be affected in a third nerve palsy.

**Option D:** This option is incorrect because it does not specifically address the restricted eye movements and ptosis without squint or diplopia. While other conditions like thyroid ophthalmopathy can cause similar symptoms, they typically involve additional features like exophthalmos or lid retraction.

**Clinical Pearl / High-Yield Fact**
Myasthenia Gravis can be a challenging diagnosis, but paying attention to the absence of diplopia and the presence of ptosis in a patient with restricted eye movements can help narrow the differential diagnosis.

**Correct Answer:** C.