Introduction / Context:
This question tests complement requirements after verbs like “require.” The sentence is split into parts; pick the segment with the error.
Given Data / Assumptions:
- “Require” followed by “to” must take a bare infinitive verb (e.g., “to attend”).
- The sentence describes an ongoing course.
Concept / Approach:
“Is required to” is incomplete without a following verb. It needs an infinitival complement such as “undergo,” “attend,” or “complete.”
Step-by-Step Solution:
A: “He has been undergoing” — correct present perfect progressive.B: “the special training course” — correct noun phrase.C: “which each of the employees” — correct relative-clause beginning; number agreement okay (“each … is”).D: “is required to.” — erroneous because it lacks the infinitive verb; should be “is required to undergo” or “is required to attend.”
Verification / Alternative check:
Corrected version: “He has been undergoing the special training course which each of the employees is required to undergo.”
Why Other Options Are Wrong:
A, B, and C are structurally fine; only D is incomplete.
Common Pitfalls:
Stopping a “to”-infinitive after “required” without the main verb.
Final Answer:
is required to.
Discussion & Comments