The correct answer is D. No error.
The sentence is grammatically correct. The word “about” is used to indicate the topic of a discussion, and the word “thoroughly” means in great detail. The word “eve” means the day or period of time just before an event, and the word “work out” means to solve a problem. The sentence is saying that we discussed the problem in great detail on the day before the examination, and that I found it very easy to solve the problem.
Option A is incorrect because the word “about” is necessary to indicate the topic of the discussion. Option B is incorrect because the word “on” is necessary to indicate the time of the discussion. Option C is incorrect because the word “that” is necessary to introduce the clause that states the result of the discussion.