Some educators claim that it is best that school courses cover only

The educators in the passage argue that "if student achieve a solid grasp of the basic concepts and investigatory techniques in a subject, they will be able to explore the breadth of that subject on their own after the course is over."

Analyzing a few Greek tragedies in detail, as (D) suggests, might not give the students a complete grasp of the techniques needed to analyze another Greek tragedy. However, if they achieve a solid grasp of these techniques, it will make it easier for them to explore and understand the breadth of other tragedies when they are reading by themselves after they finish school.

The reason it's ok for the word "any" to be used in (D) is that the student doesn't actually have to be able to understand the tragedy fully -- by analyzing a few tragedies in school, they will find it easier to understand any other Greek tragedy, so we don't need to limit our hypothetical student to understanding "most" tragedies.

You also need to be careful that you don't change any of the wording in the passage or the answer choices -- (D) says that a student will find it easier to understand (not analyze) any Greek tragedy after they have analyzed a few in detail. This seemingly tiny change in wording can have a big effect on how you look at this answer choice.

Looking at the other answer choices, none of them suggest that learning a small number of concepts and techniques in depth while at school will make it easier to explore the depth of a subject later, so you should cross out the other answer choices and (D) should be your answer to this one.

I hope that helps!