This question is asking you to find the answer choice which is an assumption that the argument relies on. I always like to start this type of question by breaking down the argument into its component parts:
Premise 1: Some gardening books by Garden Path Press recommend tilling the soil and adding compost before starting to garden, but they don't explain hot v. cold composting. Premise 2: Any book that recommends adding compost is flawed if it doesn't explain the basics of composting. Premise 3 (what's being assumed):??? Conclusion: Some books by Garden Path Press are flawed.
Then, see if you can spot the assumption before turning to the answer choices. I seem to see a gap between the conclusion and premises. Premise 1 tells us that the Garden Path books don't explain hot v. cold composting, then premise 2 tells us that a book is flawed if it doesn't explain the basics of composting, and the conclusion says this means the Garden Path books are flawed. But, is hot and cold composting a "basic of composting?" We don't know if it is yet.
Answer Choices: (C) is correct because it spots the assumption we identified above. The conclusion is assuming that hot v. cold composting is a basic of composting, but the premises have not told us that. So, this answer choice fills in that gap.
(E) is incorrect because we aren't looking to prove what gardening books are NOT flawed, but rather those that ARE flawed. This answer choice turns the argument around. It tells us that if the book includes the basics, then it is not flawed. In contrast, the argument says if the book doesn't include the basics, it is flawed. These are not the same statement.
