For the situation described in the sample question, the competency tested is Analysis and Problem Solving, but this is not explained to the candidate!
The best and worst cases are designated in relation to this competency and its specific method tested (for this question is Expert Knowledge). The answers are:
Best: B - your colleague should be the person most knowledgeable in the field and before anything you should take the chance to discuss with him the key issues.
Worst: A – you will avoid responsibilities and miss the opportunity to learn the file.
Option C – this could work; but you risk to miss something due to fatigue and also you will lose the opportunity to review the key issues with the expert.