Which of the following formulas represents the same geometric sequence as the formula below

It has to be a or b since putting a 1 in for n gives

If you put n-1 in for n you get

Now for any a_n we need the total sequence before it as well as the nth value.  But the total sequence before it is just a_(n-1):

so we need choice a, since the (n-1)th term times 5 is the nth term