**Core Concept**
The Apt test is a laboratory method used to differentiate between fetal and adult hemoglobin. It relies on the chemical properties of these hemoglobin types to identify whether blood is from a newborn or an adult.

**Why the Correct Answer is Right**
The Apt test employs sodium hydroxide (NaOH) to distinguish between fetal and adult hemoglobin. Fetal hemoglobin is more resistant to alkaline denaturation than adult hemoglobin. When a blood sample is mixed with sodium hydroxide, adult hemoglobin will denature and precipitate, while fetal hemoglobin will remain dissolved in the solution. This allows for the identification of fetal hemoglobin in the blood of a newborn.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because sodium hydroxide is a strong base that disrupts the structure of adult hemoglobin, leading to its precipitation.
**Option B:** This option is incorrect because hydrochloric acid (HCl) is an acid that would not effectively differentiate between fetal and adult hemoglobin.
**Option C:** This option is incorrect because potassium hydroxide (KOH) is also a strong base, similar to sodium hydroxide, and would have a similar effect on adult hemoglobin.

**Clinical Pearl / High-Yield Fact**
The Apt test is a simple and useful tool for distinguishing between fetal and adult hemoglobin, but it should not be used to diagnose hemoglobinopathies or other blood disorders.

**Correct Answer: C. Sodium hydroxide (NaOH).**