**Core Concept**
The question is testing the student's understanding of the differential diagnosis of ophthalmoplegia, which refers to weakness or paralysis of one or more extraocular muscles responsible for eye movements. The case presents with restriction of eye movements, ptosis, but no diplopia or squint, indicating a possible myopathic or neuropathic process.

**Why the Correct Answer is Right**
The presence of ptosis and restriction of eye movements in all directions, without diplopia or squint, suggests a disorder affecting the extraocular muscles or their innervation. The patient's age and lack of diplopia (double vision) point towards a myopathic process, such as myasthenia gravis or an idiopathic myopathy. However, given the absence of diplopia, the most likely diagnosis is an idiopathic myopathy, specifically congenital myasthenic syndrome or a non-specific myopathy. These conditions are characterized by weakness or paralysis of the extraocular muscles, leading to ptosis and restricted eye movements.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely a distractor, as it does not match the clinical presentation. However, without more information, it is difficult to determine what this option might be.
* **Option B:** This option might be a distractor, such as a cranial nerve palsy (e.g., III, IV, or VI nerve palsy), which would not explain the bilateral involvement and lack of diplopia.
* **Option C:** This option might be a distractor, such as a thyroid ophthalmopathy, which can cause ptosis and restricted eye movements, but typically presents with other symptoms such as exophthalmos and lid retraction.
* **Option D:** This option might be a distractor, such as a compressive lesion affecting the third cranial nerve, which would not explain the bilateral involvement and lack of diplopia.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with ophthalmoplegia, it is essential to consider both myopathic and neuropathic causes. In this case, the absence of diplopia and the presence of ptosis and restricted eye movements in all directions point towards a myopathic process, specifically an idiopathic myopathy.

**Correct Answer: C.**