The correct answer is "Subtract 3".

Please Note:
Subtraction is the operation that decreases a number by taking away from it.

Solution:
To find the correct rule, we consider the given table.

We observe that the given rule makes each number decrease. This implies subtraction.

We try subtracting 3 from each input number:

6 - 3 = 3
8 - 3 = 5
5 - 3 = 2
10 - 3 = 7

On subtracting 3 from the input numbers, we get the corresponding output numbers.

So, the rule is “subtract 3”.