Concept:In English, adverbs of manner (like “hard”) usually come immediately after the main verb, before adverbs of place and time.
Explanation:The verb here is “have been practising”.
The word “hard” describes how they practised, so it should be placed right after “practising”.
Next comes the place where they practised: “in the stadium”.
Finally, the time duration comes at the end: “all morning”.
This natural order is manner → place → time.
Option A places “hard” after “in the stadium”, which is awkward.
Options B and C put time or manner in positions that disrupt the flow.
Option D follows the correct and natural pattern: “practising hard in the stadium all morning”.
Therefore, the best completion is option D.
Answer:D. hard in the stadium all morning