Since the question doesn't state whether he wants his students to make the figure larger or smaller we can just find both.
Multiply by 3 to make the sides larger.
2 * 3 = 6 inches
3.1 * 3 = 9.3 inches
4.5 * 3 = 13.5 inches
3.9 * 3 = 11.7 inches
Divide by 3 to make the sides smaller.
2 / 3 = 0.66... inches
3.1 / 3 = 1.033... inches
4.5 / 3 = 1.5 inches
3.9 / 3 = 1.3 inches
Mark said 5 inches, so, he is incorrect.
Samantha said 6 inches, so, she is correct.
Divia said 1.3 inches, so, she is correct.
Cheng said 0.9 inches, so, he/she is incorrect.
Candice said 11.7 inches, so, she is correct.
Best of Luck!