**Core Concept**
The PEFR (Peak Expiratory Flow Rate) of a group of individuals follows a normal distribution, which is a probability distribution that is symmetric about the mean. In this case, the mean PEFR of the 11-year-old girls is 300 l/min, and the standard deviation is 20 l/min, indicating that most of the girls have PEFR values close to the mean.

**Why the Correct Answer is Right**
The normal distribution of PEFR values implies that about 68% of the girls have PEFR values between 280 l/min (mean - 1 standard deviation) and 320 l/min (mean + 1 standard deviation). This is because the normal distribution is characterized by the 68-95-99.7 rule, also known as the empirical rule. The mean PEFR of 300 l/min represents the average value, and the standard deviation of 20 l/min represents the amount of variation in the data.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot evaluate it.
**Option B:** This option is also not provided, so we cannot evaluate it.
**Option C:** This option is not provided, so we cannot evaluate it.
**Option D:** This option is also not provided, so we cannot evaluate it.

**Clinical Pearl / High-Yield Fact**
The normal distribution of PEFR values is an important concept in respiratory medicine, as it helps clinicians understand the variability in lung function among individuals. A PEFR value that falls below the 5th percentile (mean - 1.645 standard deviations) may indicate respiratory disease, such as asthma or chronic obstructive pulmonary disease (COPD).

**Correct Answer:** Not enough information to determine the correct answer from the provided options.