To obtain the position vector of the midpoint of the line PQ, the following steps are necessary:
Step 1: Recall the expression for the line AB, obtained from two position vectors A, and B, as follows:

Also:

Step 2: Recall the expression for the midpoint of the line vector PQ, as given below:

Thus:

Step 3: Apply the formula for the midpoint of line PQ, to the question, as follows:

Therefore, the position vector of the midpoint of the line PQ is : (2, 6, 7) (option B)