Final answer:
The correct conjunction to complete the sentence about Bill wanting to enter Oxford University but having trouble meeting the requirements is 'but' to indicate the contrast between his desire and the difficulties he faces.
Step-by-step explanation:
The student's question 'Bill wants to enter Oxford University but has some trouble meeting the academic requirements? 1) and 2) but 3) as 4) so' is asking to choose the correct conjunction to complete the sentence. The appropriate conjunction to use in this context is 'but', which indicates a contrast between Bill wanting to enter Oxford University and having trouble meeting the academic requirements. Thus, the sentence should read: 'Bill wants to enter Oxford University but has some trouble meeting the academic requirements.' The other provided options ('and', 'as', 'so') do not properly fit in the context of the sentence as they do not express the contrast that is implied.