Government subsidies to urban manufacturers might help ease urbanization problems, but this has nothing to do with the economists' proposed solution. The economists are proposing that we trade urban-produced goods and services for agricultural products. (A) is about a completely different approach (subsidies), so it doesn't support the economists' assertion.

Even if we accept that economic problems require economic solutions, this is far too general to support the economists' specific proposal. The economists aren't just proposing any economic solution; they're proposing a particular kind of trade. (B) doesn't give us any reason to think that this particular trade-based approach would be effective.

This strengthens the economists' assertion by connecting the problems of urbanization to a lack of agricultural products. The economists' solution is specifically designed to obtain agricultural products through trade. If agricultural scarcity is a central element of the problems caused by urbanization, then a plan that addresses that scarcity by trading for agricultural products has a much better chance of actually solving those problems. Without this link, we'd have no reason to think that obtaining agricultural products would help.

(D) tells us that the problems of urbanization are primarily caused by trade imbalances between countries. But the economists' solution is to trade urban goods and services for agricultural products. That's a solution aimed at obtaining agricultural products, not a solution aimed at correcting trade imbalances. Even if trading for agricultural products happens to involve international trade, we don't know whether that trade would reduce the imbalances or make them worse.

If free trade policies can make urbanization problems worse, that gives us reason to doubt that a trade-based solution would be effective. The economists are proposing more trade, not less, so (E) would, if anything, weaken their position.