A deposit of $25 is placed in a savings account. Each month, the amount deposited is quadrupled. Write the function that models the exponential change between the number of months, (t), and the amount of money deposited in the savings account, (a(t)).

a) Express the initial deposit of $25 in the function.
b) Formulate the exponential growth function in terms of the quadrupling monthly deposit.
c) Define the variables in the function and their significance.
d) Provide the complete exponential function (a(t)).