**Core Concept**
The underlying principle being tested is the pathophysiology of autoimmune thyroid disease, specifically Graves' disease, characterized by autoantibodies against the thyroid-stimulating hormone receptor, leading to excessive thyroid hormone production.

**Why the Correct Answer is Right**
The patient's symptoms, including excessive nervousness, irritability, and exophthalmia, are classic signs of hyperthyroidism. The presence of a goiter and high antibody titers against the TSH receptor are diagnostic hallmarks of Graves' disease. The autoantibodies stimulate the TSH receptor, mimicking the action of TSH and leading to increased thyroid hormone production. This results in the characteristic symptoms of hyperthyroidism.

**Why Each Wrong Option is Incorrect**
* **Option A:** Toxic multinodular goiter is a type of thyroid cancer, which would not present with high antibody titers against the TSH receptor.
* **Option B:** Hashimoto's thyroiditis is an autoimmune thyroid disease, but it typically presents with hypothyroidism, not hyperthyroidism, and is characterized by antibodies against thyroid peroxidase, not the TSH receptor.
* **Option D:** Subacute thyroiditis is an inflammatory condition of the thyroid gland, which would not present with high antibody titers against the TSH receptor.

**Clinical Pearl / High-Yield Fact**
Graves' disease is the most common cause of hyperthyroidism and is often associated with ophthalmopathy, which can lead to exophthalmia and other eye symptoms. Recognizing the classic symptoms and laboratory findings is crucial for diagnosis.

**Correct Answer:** C. Graves' disease.