Why is D correct? Why is B incorrect?

The mistake you may have made with this one and chose B is failed to read the entire question stem. If you only applied the PT block and not the PTW, then B would be an attractive answer. But if you apply the entire condition PTW you realize T can not be in the-3rd position for that would put P in 2, leaving just position 1 for U (U-before-P) and Q( Q-before-W), therefore R must be in position 3. with the PTW block coming after the RS Block, therefore answer choice (D) "R must be before P" is the correct answer. The mistake I often make is rushing and reading half the question and assuming; you know what they say about assuming.....