Solution

To determine the average access time using binary search in a fixedlength record file ordered by the key field, we need to understand how binary search works on disk blocks.

In a binary search, the search space is halved after each comparison.
The total number of blocks is B.
Binary search will typically require \(\log_2 B\) comparisons to locate the correct block.

Thus, the average access time is proportional to the number of comparisons made to find a record in the file, which is approximately \(\log_2 B\) disk accesses.

Therefore, the correct answer is: 4) \(\log_2 B\).