How many 6's are there in the following sequence each of which is NOT immediately preceded by 3 but immediately followed by 3?
8 6 3 6 4 8 6 8 1 8 6 3 6 5 4 8 6 2 4 3 6 8 6 9 8 8 6 4 6 9

Given:  8 6 3 6 4 8 6 8 1 8 6 3 6 5 4 8 6 2 4 3 6 8 6 9 8 8 6 4 6 9

Condition: 

6 must not be immediately preceded by 3.

6 must be immediately followed by 3.

The 6 at position 2 is not preceded by 3 (it’s preceded by 8).

The 6 at position 11 is not preceded by 3 (it’s preceded by 8).

Both occurrences meet the criteria that:

They are followed by 3.

They are not preceded by 3.

Thus, the correct answer is indeed two occurrences.

The correct option is (a) Two.