Final answer:
The length of bar (Wx) in the similar trapezoids is 17, so the correct answer is B. 21.
Step-by-step explanation:
To find the length of bar (Wx), we can use the concept of similarity in trapezoids. Similar figures have proportional sides. Let's say that the length of Wx in the smaller trapezoid is 'x', and the length of Wx in the larger trapezoid is 'y'. Since the trapezoids are similar, we can set up the following proportion:
x / 4 = y / 8.5
Cross-multiplying and solving for y gives us y = 17. Therefore, the length of Wx is 17, so the correct answer is B. 21.