Sometimes the easiest thing to do is test the hypotheses.
In order for A to be correct, when we plug 1 in for t, we should get 8.
2.5^(1-1) = 2.5 ^ 0 = 1, so yes. In year one, the coin is valued at $8.
We have our answer, but for kicks let's do the rest.
For B, if we pick the year 2, the value should then be 16.
2.5 ^ (2-1) = 2.5 ^ 1 = 2.5. 8*2.5 = 20.
For C, there is no reason to infer number of coins. It has nothing to do with the question being answered.
For D, we look at the answers to A and B again. Is B 8 times A?
So, the only answer which comes back correct is A.