Refer to the following number series and answer the question.

7 2 9 6 3 7 3 9 7 7 3 9 73 2 2 1 3 3 9 3 7 39 3 9

How many 3s are there that are immediately preceded by 7 and immediately followed by 9?

So, there are such three pairs which 3s are there that are immediately preceded by 7 and immediately followed by 9

as shown below.

Thus, Option D is the correct answer.