2017 AMC 10A Problem 2

The $\$3$ boxes give us the most popsicles per dollar, so we want to buy as many of those as possible. We can buy two of those, getting $5 \cdot 2 = 10$ popsicles with $\$8 - \$6 = \$2$ remaining. The $\$1$ single popsicles are the worst deal, so Pablo should spend the rest of his money on the $3$-popsicle box. He then ends up with $10 + 3 = 13$ popsicles. Thus, D is the correct answer.