The correct answer is: D. a long way off.
The phrase “a long way off” means that something is not going to happen soon or is very unlikely to happen. In the context of the question, it means that world peace is not going to happen anytime soon, despite the efforts of all peace-loving people.
The other options are incorrect because they do not accurately reflect the meaning of the phrase “a long way off.” Option A, “an impracticable idea,” means that something is not possible or feasible. Option B, “an abstract idea,” means that something is theoretical or not based in reality. Option C, “out of reach,” means that something is very difficult or impossible to achieve.
It is important to note that the phrase “a long way off” is subjective. What one person considers to be a long way off may not be considered to be a long way off by another person. Additionally, the phrase “a long way off” can be used in a variety of contexts, and its meaning can vary depending on the context.