Helena did not recognize her English teacher when she unexpectedly saw him while traveling in Paris, even though she knew him well back in the classroom. The fact that Helena can recognize her teacher back home more easily than in Paris best demonstrates what concept?

The answer is perceptual set.

Explanation:

In simple words, this concept states that we tend to perceive our environment in a limited, personal way. That means that our view of the world is influenced by our experiences, culture, emotions and expectations.

In the example, Helena perceives the teacher and the classroom as a sort of unity, probably because she had never though about her teacher in a different context.