In these lines, Swift is being ironic. By dismissing these practical solutions as "impractical," he is proving that people already had the tools to help the poor without going to extremes.
Also, by calling smart solutions "impratical," Swift further highlights the absurdity of his own solution (selling and eating young children).
Therefore, by dismissing legitimate solutions as "impractical," he is actually proving how practical they are.
The answer is D. To be able to remember the important parts of what you just read.