​How many 7s are there in the following series, each followed by a 1 and preceded by a 5?
2 7 3 5 7 1 5 7 3 5 7 1 9 7 3 5 7 1 7 3 6 5 7 1 8 1 4 7 3 5​

Given: 2 7 3 5 7 1 5 7 3 5 7 1 9 7 3 5 7 1 7 3 6 5 7 1 8 1 4 7 3 5

So, there are four, 7s in the following series, each followed by a 1 and preceded by a 5.

As, shown above.

Thus, Option B is the correct answer.