**Core Concept**
The mean is a statistical measure used to describe the average value of a dataset. It is calculated by summing all the values and dividing by the total number of values. In this case, we need to find the mean of the nicotine levels in 6 cigarettes.

**Why the Correct Answer is Right**
To calculate the mean, we need to add up all the values and divide by the total number of values. Let's do the math:

13.0 + 11.8 + 15.4 + 17.4 + 31.2 + 24.3 = 113.1

There are 6 values in total. To find the mean, we divide the sum by 6:

113.1 ÷ 6 = 18.85

So, the mean nicotine level in the 6 cigarettes is 18.85 mg.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not the correct calculation of the mean. Let's assume it was a wrong calculation, which would result in a different value.
* **Option B:** This option is incorrect because it does not take into account the correct calculation of the mean. Without the correct sum, this option would be incorrect.
* **Option D:** This option is incorrect because it does not match the correct mean value of 18.85 mg.

**Clinical Pearl / High-Yield Fact**
When calculating the mean, it's essential to ensure that you have the correct sum of the values and divide by the correct number of values to avoid errors.

**Correct Answer:** A. 18.85