Whether and if are both conditional linking words. In this case, they would suggest that Henry standing on his head might be what causes the box to be placed upside down. This is not the case, so whether and if are not correct. The use of although would suggest a contradiction -- that the box was unlikely to be placed upside down if Henry was standing on his head. However, the people moving the box didn't know that Henry was inside, so this answer doesn't make sense either. Because would also suggest that people moving the box knew that Henry was inside, and what position he was in, so because is incorrect too. And is the correct answer.