**Core Concept**
To compute the median of a set of data, we need to find the middle value when the numbers are arranged in ascending order. The median is an important measure of central tendency, especially when the data set is small or has outliers.

**Why the Correct Answer is Right**
The given set of data, 1, 2, 3, 4, 5, 6, has an even number of values (6), so the median will be the average of the two middle values. When arranged in ascending order, the middle values are 3 and 4. To find the median, we take the average of these two values: (3 + 4) / 2 = 3.5. This is the correct median for the given data set.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the median is not simply the middle value, especially when the data set has an even number of values. We need to take the average of the two middle values.

**Option C:** This option is incorrect because it is one of the middle values, but not the correct median when taking the average of the two middle values.

**Option D:** This option is incorrect because it is not a possible median for the given data set. The median must be a value within the data set, and 4.5 is not present in the data.

**Clinical Pearl / High-Yield Fact**
When computing the median for a data set with an even number of values, always remember to take the average of the two middle values. This is a simple yet important concept in statistics.

**✓ Correct Answer: B. 3.5**