We should notice a few differences right away:

First, the verb at the end of the sentence switches between is and are. C, D, and E matches singular "effort" with singular "is", B matches plural "effort" with plural "are", while A has singular "effort" and plural "are". So we can eliminate A for agreement issues.

We should also notice that C uses "involved with" rather than "involved in". "Involved with" is a different idiom with a different meaning - a person can be involved with another person or an organization or a cause, but effort cannot be involved with memorizing. So we can eliminate C for idiom issues.

Looking at B, D, and E, we see that both B uses the phrase "in the memorizing of", D uses "in the memorization of", while E using "in memorizing". We'll start with B. Looking at the rest of the sentence, we see there are two things that effort is involved in:

the efforts involved in
-the memorizing of endless vocabulary words
-doing countless drills

Because we have a list of two things, they must be grammatically parallel. But they aren't in B! Looking at D, we see the the same thing:

the effort involved in
-the memorization of endless vocabulary words
-doing countless drills

However, in E, we see:
the effort involved
-in memorizing endless vocabulary words
-in doing countless drills is daunting

So because B and D aren't parallel OR concise (and because there is still no clear reason for "effort" to change to "efforts", we can rule out B and D and pick E.