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-->IMO correct
(b) A requirement, according to standard police procedure, is that an officer
interview witness separately rather than
interviewing them together-->not //
(c) Standard police procedure requires an officer to interview witnesses separately rather than to interview them together--> i can think of the grammar wrong here, i can see only that A is more concise
(d) It is required in standard police procedure that an officer
interviews witnesses separately, not together
(e) Standard police procedure requires that an officer
interviews witnesses separately rather than together
"Require" is subjunctive V. Therefore, the verb after "require to" or "require that" has to be bare form verb (no -s, no -ed)--> eliminate E,D