The least value of 'n' for the recursion would be 3.
For n = 1, number of strings = 2 (0, 1)
For n = 2, number of strings = 3 (00, 01, 10)
For n = 3, number of strings = 5 (000, 001, 010, 100, 101)
For n = 4, number of strings = 8 (0000, 0001, 0010, 0100, 1000, 0101, 1010, 1001) ...
This seems to follow Fibonacci series and the recurrence relation for it is a_{n} = a_{n−1} + a_{n−2}. Thus, B is the correct choice.