Gary needs 24screws for every 2 chairs he builds.

D

Step-by-step explanation:

First put the data from smallest to largest

12 points, 17 points,21 points,  23 points, 24 points, 28 points,29 points

The left end of the whisker should be 12  (This eliminates graph a)

The right end of the whisker should be 29  (This eliminates graph b)

There are 7 points

The middle point is 4

The 4th point is 23

That is the line in the middle of the box  (This eliminates graph c)

The lower line of the box is at at point 2 = 17

The upper line of the box is at  point 6 = 28

This leaves us with graph D