Consider the following page reference string. 1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2…

Reference String: 1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6

Distinct pages = {1, 2, 3, 4, 5, 6, 7}

To obtain exactly one additional page fault after the initial loading phase:

With 5 or fewer frames, multiple replacements occur, causing several page faults.
With 6 frames, pages 1–6 can be accommodated. When page 7 is referenced, it causes exactly one replacement under LRU. After that, the remaining references are hits.
With 7 frames, all distinct pages fit in memory, so no additional page fault occurs after the initial loading.
Therefore, the minimum number of frames required is 6.

Answer: C) 6