Correct Answer - Option 4 : 8
Here, the digits of the first number in the series are multiplied to get the second number, then the digits of the second number are multiplied to get the third number and so on.
Therefore, the pattern followed here is :
9 × 4 = 36
3 × 6 = 18
1 × 8 = 8
Hence, 8 is the correct answer.