Step 1: When two subjects are joined by as well as, the verb agrees with the first subject.
Step 2: The correct sentence should be: The teacher as well as his students left for the trip.
Step 3: The word all is unnecessary and makes the construction incorrect. Hence, part (b) contains the error.
Identify the part of the sentence that contains an error:
"Neither the manager nor the employees ___ willing to accept the new policy."