At what BMI value is a person considered underweight?

A person is considered underweight when their Body Mass Index (BMI) is 18.5 or below. The BMI is a widely used indicator that helps categorize individuals based on their body weight relative to their height. This classification is essential for understanding health risks associated with weight.

The threshold of 18.5 acts as a cutoff point indicating that a person may not have enough body mass to maintain optimal health. Being underweight can lead to various health issues, including weakened immune function, osteoporosis, and fertility problems, among others. Therefore, the determination of being underweight requires BMI to be 18.5 or less, which supports the conclusion that underweight individuals fall into the category of having a BMI of 18.5 and below.

Other options do not accurately represent the classification for being underweight, as they either indicate a higher BMI or do not establish a clear range. Hence, option C is the only accurate choice for defining underweight based on BMI standards.