This question already has 3 great answers, but all seem oriented towards more advanced students. OP doesn't specify the grade level of the students in the question, but this approach might be more suitable if they're younger learners. It is a method which helped me when I was elementary/middle school aged:
Here is the sentence from the question:
"In one study, 43% of employees ____ as being under heavy stress had weak concentration and poor work performance."
If the students are confused as to which choice ("are described" vs. "described") should go in the blank, I would assume they aren't clear on either the intended meaning of the sentence, or how the sentence's structure conveys that meaning.
To clear up the confusion, it might be helpful to have the students strip down the sentence to the most basic elements, then build it back up. Maybe ask something like, "Can you write down just the subject, main verb, and direct object of the sentence - think of who did what? Ignore all the modifiers (adjectives, adverbs, prepositional phrases, etc.) for now."
This would give "employees (S) had (V) concentration and performance (D.O.)."
Then, encourage them to add the modifiers back in, one at a time: what kind of concentration and performance ("weak concentration and poor performance")? How many employees ("43% of employees")? etc.
Once they have reconstructed the original sentence and identified which central part (Subject, Main Verb or Direct Object) each modifier adds more information to, ask them to think about which of the two options "are described" or "described" could go in the blank without changing the meaning of the sentence. We've already identified S, V and D.O., so adding another main verb ("are described") in the blank would change the meaning. (Now you'd have "employees (S) are described (V)," and the sentence would fall apart as soon as they try to find where the Dir. Obj. went or how the word "had" fits into the new sentence.)
But, with "described" in the blank, they should now be able to identify it as yet another modifier for the subject of the sentence, "employees". It answers the question, "what kind" of employees - the "described-as-being-under-heavy-stress" kind!
Once the students have gotten past any confusion on how the parts of the sentence fit together (I believe the above exercise should help with that), would be the time to go into more detail about technical terminology like how exactly you'd classify the phrase "described as being...".