The correct answer is Option A:

Solution:
Considering the given options.

Option A.   

Option B.

On Option A:  Considering the given number pattern.

Here, the numbers are decreasing.
So, we first find the rule.

Note that:
18 - 3 = 15
15 - 3 = 12
12 - 3 = 9
9 - 3 = 6

So, the rule is "subtract 3", and this is the required answer.

Hence, the correct counting pattern with the rule ‘subtract three’ is Option A: