In each of the following questions, select the option that best completes the gap. He went ____ the corridor, looking for the principal's office.

To determine the best option to complete the sentence, we need to consider the direction and context provided by the sentence. The sentence is describing the action of "going" in relation to the "corridor". Let's analyze each option:

• Under: This option implies moving beneath something. "Under the corridor" would suggest moving below the corridor, which doesn't make sense in this context.
• Among: This option suggests being in the midst of or surrounded by something. "Among the corridor" is not logical because a corridor typically doesn't have multiple objects to be surrounded by; it's a passageway.
• Along: This implies movement in the same line or direction as an object. "Along the corridor" makes sense because it suggests moving in the direction of the corridor towards the principal's office.
• Around: This signifies movement in a circular or surrounding manner. "Around the corridor" would suggest moving around the corridor's perimeter, not through it, which is not the intended direction in this context.

In conclusion, the best option is "along" because it accurately describes moving in the same direction as the corridor while looking for the principal's office.