Remark
The closest answer is D, but the answer has to be rewritten: f(x) = - 2/3 (6)^(x - 1) where x is 1,2,3,4,5,6 .... to plus infinity. If that is not what you are given, there is no answer.
Discussion
The minus sign is misplaced. There are 2 reasons.
1. When x is odd, then the minus sign is not there and every other term will be plus.
2. For a related reason, you cannot get a minus 2/3 if the sign is affected by the power. The first term must be -2/3. (-6)^0 power is 1 not -1
Answer: There is no answer, but you could try C if D does not work.