Which of these brand names has NOT become a genericized trademark in American English, still maintaining its exclusive brand identity without becoming a generic term for its product category?

In the fascinating intersection of linguistics and marketing, some brand names have been so successful they've actually entered our everyday vocabulary as generic terms. This linguistic phenomenon, called 'genericide,' can be both a triumph of brand recognition and a potential trademark risk. Test your knowledge about these powerful examples of brand names that transcended their product identity to become part of our lexicon.

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Kleenex (facial tissues)
Fitbit (activity trackers)
Thermos (vacuum flasks)
Escalator (moving staircases)