**Core Concept**
Acute malnutrition in children is a critical public health concern, and identifying the best indicator is essential for early detection and intervention. The World Health Organization (WHO) and other international health organizations recommend the use of anthropometric measurements to assess malnutrition in children.

**Why the Correct Answer is Right**
The correct answer is the Mid-Upper Arm Circumference (MUAC). MUAC is a simple, cost-effective, and non-invasive method to assess the nutritional status of children aged 6-59 months. It measures the circumference of the mid-upper arm, which is a reliable indicator of muscle mass and fat stores. A MUAC measurement of less than 11.5 cm is considered indicative of acute malnutrition. This measurement is particularly useful in resource-limited settings where other anthropometric measurements may not be feasible.

**Why Each Wrong Option is Incorrect**
**Option A:** Weight-for-height is also an anthropometric measurement, but it is not the best indicator of acute malnutrition in children. Weight-for-height is more sensitive to acute changes in weight, whereas MUAC is more reflective of overall nutritional status.
**Option B:** Height-for-age is a measure of stunting, which is a indicator of chronic malnutrition. It does not accurately reflect acute malnutrition.
**Option C:** Body Mass Index (BMI) is not a suitable indicator for acute malnutrition in children, as it is based on weight and height, which may not accurately reflect muscle mass and fat stores.

**Clinical Pearl / High-Yield Fact**
When assessing malnutrition in children, it's essential to remember that MUAC is a sensitive indicator of acute malnutrition, and a measurement of less than 11.5 cm requires immediate attention and referral to a healthcare provider.

**Correct Answer:** C.