Page Reference Sequence: 1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
Number of Frames: 3

Explanation:

The Optimal page replacement algorithm is an algorithm that selects the page for replacement which will not be used for the longest duration of time in the future. To determine the number of page faults, we need to simulate the algorithm by maintaining three frames.

Initially, all three frames are empty. As we process each page in the sequence, we check if the page is already present in one of the frames. If it is present, we move to the next page.

If the page is not present in any frame, we need to replace one of the existing pages with the current page. The replacement is done based on the future reference of pages.

Let's simulate the Optimal page replacement algorithm for the given page reference sequence:

| Page | Frame 1 | Frame 2 | Frame 3 | Page Fault? |
|---------|---------|---------|---------|-------------|
| 1 | 1 | | | Yes |
| 2 | 1 | 2 | | Yes |
| 3 | 1 | 2 | 3 | Yes |
| 4 | 4 | 2 | 3 | Yes |
| 2 | 4 | 2 | 3 | No |
| 1 | 4 | 2 | 3 | No |
| 5 | 4 | 2 | 5 | Yes |
| 6 | 4 | 2 | 5 | Yes |
| 2 | 4 | 2 | 5 | No |
| 1 | 4 | 2 | 5 | No |
| 2 | 4 | 2 | 5 | No |
| 3 | 4 | 3 | 5 | Yes |
| 7 | 4 | 3 | 7 | Yes |
| 6 | 4 | 3 | 7 | Yes |
| 3 | 4 | 3 | 7 | No |
| 2 | 4 | 3 | 7 | No |
| 1 | 4 | 3 | 7 | No |
| 2 | 4 | 3 | 7 | No |
| 3 | 4 | 3 | 7 | No |
| 6 | 6 | 3 | 7 | Yes |

Total Page Faults: 11

Therefore, the correct answer is option 'A' - 11.