how would the equation be written if the interest rate had been 4% per year?

Hello.
Firstly, 4 percent of 100 is 0.04 percent, and given the equation y = 3000(1 + 0.03)^n, you replace the 0.03 with 0.04.
Therefore, the answer is C, y = 3000(1 + 0.04)^n.