That formula will never give a negative term no matter what 
positive whole number we substitute for n.  D is the only 
choice that has only positive terms, so D is the only possible 
correct choice. 

But let's suppose we didn't notice that:

We'd plug in 1 for n


    <-- that rules out B, the others have 1st term 1/2

We'd plug in 2 for n


    <-- that rules out A, for C & D have 2nd term 1/4

We'd plug in 3 for n


    <-- that rules out C.

So the answer must be D, but let's check to make sure:

We plug in 4 for n


    <-- yes, D is the correct choice.

Edwin