Answer D is correct. You know that 25,000 is 50% of 50,000, so filling the page by 50% and leaving 50% free space for the remaining 50,000 records seems logical. The default FILLFACTOR setting of 0 doesn't leave any room for additions, which would slow inserts. If you set the FILLFACTOR value too big, searches slow down because any query processed has to cycle through a lot of empty space.
For more information, see the following sources: