B and C are absurd; if a series converges, it must have a sum, but if a series diverges, it cannot have a sum.
Now, notice that
That is, we can write the sum more compactly as
The series is geometric with common ratio
, so the series converges (and thereby has a sum), so the answer is D.
Now, notice that
That is, we can write the sum more compactly as
The series is geometric with common ratio