Select the correct option:
Neither the principal investigator ___ the co-authors anticipated the backlash.

Correct option is A

The correct substitution of the highlighted words is (a) nor.
In the correlative conjunction pair neither…nor, we use nor to join the second subject/clause. Here the sentence has two noun phrases—“the principal investigator” and “the co-authors”—so it must be: Neither A nor B + verb. Hence, “Neither the principal investigator nor the co-authors anticipated…” is grammatically correct.
Grammatical rule used:
· Neither…nor is used to connect two alternatives in a negative sense.
· The verb usually agrees with the nearest subject (proximity rule): Neither the teachers nor the principal is … / Neither the principal nor the teachers are …
Example: Neither the manager nor the staff members were ready for the inspection.
Information booster / exception:
· “Nor did” is used in inversion when “nor” begins a new clause: He didn’t apologize, nor did he explain. That structure is not needed here because the blank is inside the fixed pair neither…nor.