Which of the following is the algebraic expression that best describes the sequence 3,6,9,12 ?

I don't see any choices so I am going to give you what I think the answer would be. The rule would be 3x. That would be you could plug any number in and they would give you the sequence. Example: 
x = 1
x times 3 = 3.
We could even find future things like:
x = 10
x times 10 = 30.
If we were doing it based on the algebraic expression it would be "multiplication" because we are multiplying. Hope this helped. :)