Standard police procedure requires that an officer interview witnesses separately rather than together.
(a) Standard police procedure requires that an officer interview witnesses separately rather than together => Answer IMO. Use of subjunctive mood is grammatically correct here.
(b) A requirement, according to standard police procedure, is that an officer interview witness separately rather than interviewing them together => not sure if grammatically correct, but in this sentence emphasis is on "a requirement", original sentence talks mainly about "standard police procedure" so OUT
(c) Standard police procedure requires an officer to interview witnesses separately rather than to interview them together => although seems grammatically correct, i think "to interview them together" is wordy. so OUT.
(d) It is required in standard police procedure that an officer interviews witnesses separately, not together => Passive voice not required. awkward. OUT.
(e) Standard police procedure requires that an officer interviews witnesses separately rather than together => Interviews is incorrect. if subjunctive mood use root form of the verb, OUT.

can any expert please verify my reasoning here please? much appreciated. thanks.

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The correct construction, as in (A), is "Requires that X Y," where X is a noun and Y is the root form of the verb. (E) is the common trap: it seems right because "interviews" is conjugated with "officer," but in fact there is no subject-verb agreement, because this is all in the Subjunctive Mood, coming after the Directive Verb "requires." We must use the root form, "interview," regardless of the apparent subject of the verb. The root form is what you'd find in the infinitive, such as "be" from "to be" or "go" from "to go" or "interview" from "to interview." Thus:

This question requires that you be aware of the subjunctive rule with directive verbs.

The law requires that anyone who is caught littering go to jail.

(B) awkwardly rephrases the sentence to "a requirement is that...," and more importantly, makes a major parallelism error in "to interview ... rather than interviewing."

(C) is the other correct way to use a directive verb: dropping the "that" and using the entire infinitive. However, sumgb is correct: we don't need to repeat "to interview." Not only is it wordy (unnecessary repetition), it's confusing since the main contrast is separately vs. together, not interview. vs. interview.

(D) and (E) both use "interviews" with "required," and in neither case is this acceptable.