**Core Concept**
Fine-needle aspiration cytology (FNAC) is a diagnostic tool used to evaluate thyroid nodules, particularly discrete thyroid swellings. It involves inserting a thin needle into the nodule to collect a sample of cells for histopathological examination, helping to differentiate between benign and malignant lesions.

**Why the Correct Answer is Right**
FNAC is the investigation of choice for discrete thyroid swellings because it is minimally invasive, highly sensitive, and specific for diagnosing thyroid malignancies. The procedure is usually performed under ultrasound guidance to ensure accurate targeting of the nodule. FNAC helps to identify specific cytological features of thyroid lesions, such as papillary fronds, follicular patterns, or the presence of lymphocytes, which can guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is useful for characterizing thyroid nodules, but it cannot differentiate between benign and malignant lesions. Ultrasound is often used in conjunction with FNAC but is not the primary diagnostic tool for discrete thyroid swellings.
**Option B:** CT scan is not typically used for evaluating discrete thyroid swellings, as it provides less detail than ultrasound or FNAC. CT scans are more useful for larger thyroid masses or when assessing for distant metastases.
**Option C:** MRI is not the primary imaging modality for thyroid nodules. While it can provide detailed images of the thyroid gland, MRI is generally not necessary for evaluating discrete thyroid swellings.
**Option D:** Biopsy is a more invasive procedure than FNAC and is usually reserved for cases where FNAC results are inconclusive or suggestive of malignancy. Biopsy is not typically the first-line investigation for discrete thyroid swellings.

**Clinical Pearl / High-Yield Fact**
Remember that FNAC is not a definitive diagnostic tool and should be interpreted in the context of clinical findings, patient history, and imaging studies. A repeat FNAC or further diagnostic workup may be necessary if the initial results are inconclusive or suggestive of a complex lesion.

**Correct Answer: C. FNAC**