E is a pretty attractive answer, it tells us that if a mandate were introduced, then producers could find substitutes. However, it tells us nothing about the cost or profitability of those substitutes. The claim that the argument is contradicting is that it would be extremely expensive to replace fossil fuels. Just because there are alternatives does not mean that it would be economical to use them. D clarifies that the cost of switching would not be too high, while E only tells us that there are alternatives to switch to- but does not rule out the idea that these alternatives are prohibitively expensive.