**Core Concept**
A multinodular goitre is a type of non-toxic goitre characterized by the presence of multiple nodules in the thyroid gland, often resulting from chronic stimulation of the thyroid gland. The sudden increase in size and hoarseness of voice suggest compression or invasion of adjacent structures, in this case, the recurrent laryngeal nerve.

**Why the Correct Answer is Right**
The hoarseness of voice is likely due to compression or invasion of the recurrent laryngeal nerve, which is at risk in cases of thyroid malignancy. The sudden increase in size of the multinodular goitre could be indicative of a thyroid cancer, such as papillary or follicular thyroid carcinoma. These cancers can arise from the pre-existing nodules in a multinodular goitre and can cause sudden enlargement due to rapid cell growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be considered if the patient had a history of radiation exposure or a family history of thyroid cancer, but there is no mention of these risk factors in the scenario.

**Option B:** A thyroid abscess would typically present with signs of infection, such as fever and redness, which are not mentioned in the scenario.

**Option C:** A subacute thyroiditis would typically present with painful thyroid gland, fever, and elevated inflammatory markers, which are not mentioned in the scenario.

**Clinical Pearl / High-Yield Fact**
A sudden increase in size of a multinodular goitre, especially in the presence of hoarseness of voice, should raise suspicion for thyroid malignancy, and prompt further evaluation with imaging and fine-needle aspiration cytology.

**Correct Answer:** C. Thyroid malignancy.