This sudden onset of anxiety experienced by Kayleigh over a stray dog best illustrates spontaneous recovery.
Spontaneous recovery refers to the sudden reappearance of a previously extinct conditioned response (feeling of anxiety) long after the unconditioned stimulus (fear of stray dogs) has disappeared or been removed or forgotten.
While Kayleigh could have imagined that her fear of stray dogs had disappeared, her response to being confronted by one could cause her to recover her former fears.
Thus, this sudden onset of anxiety experienced by Kayleigh over a stray dog best illustrates spontaneous recovery.
Learn more about spontaneous recovery at https://brainly.com/question/19606017
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