The sentence is meaningful in this order: (D), (A), (C), (B)
(D) She worked on the case. (A) for seven hours but was hardly (C) able to decide who the murderer was (B) because of the absence of any motivation.
Explanation: This sentence describes someone working hard on a case but struggling to solve it due to a lack of motivation or clear evidence. The correct order provides a logical flow of actions, starting with the work on the case, the duration of effort, the inability to make a decision, and the reason for the difficulty in solving the case.