Remark: It definitely is not the last one. 1/[2*(1 - 1)] = undefined (1/0 is undefined.)
C can be eliminated on sight. The denominator will be an even number. That's because k+1 is multiplied by 2 which is will always be even.
A is your second best answer. The problem is that it will never give 1 as answer. The sequence will start with 1/(2*1 + 1) = 1/3
B is the answer. 1/(2*1 - 1) = 1/(2 - 1) = 1/1 = 1. You have the right answer.