Answer: D

Say that the length of each side of the poster is x. Then, the area is x^2. If each side were doubled, each side would be 2x. The area would now be (2x)^2 = 4x^2. To find the percent increase, divide the change over the original:
(4x^2 - x^2) / x^2
3x^2 / x^2
3/1 = 300%, choice (D).