Explanation:
Choice A.
Choice A is exponentially because the amount is double each day. In other words each day the amount gets larger. One day it increased by 10 cents, the next day 20 cents, the next day 40 cents and so on... eventually it'll be a day where it increases by a million cents.
Choice B is not exponentially, it is linearly because each day the amount increase stays the same. One day is $5.00 more, the next day it's $5.00 more, the next day it's $5.00 more... and so on.
Because of that, we can get rid of those two choice B options.
Both Choice A's look the same. The only difference is that one has a 2 and the other doesn't. That 2 indicates that we're doubling. Makes sense because that's what we associate with doubling.
Making the second Choice A the correct formula.