a) y = e^(2x)
b) y = e^(-2x)
c) y = sin(2x)
d) y = cos(2x)
Answer: c) y = sin(2x) and d) y = cos(2x)
Explanation: The characteristic equation of the differential equation is r^2 + 4 = 0, which has roots r = ±2i. Therefore, the general solution is y = c1sin(2x) + c2cos(2x), where c1 and c2 are constants. Therefore, both c) and d) are solutions.