After a scary biking accident, alex extinguished his conditioned fear of bikes by cycling on a safe biking trail every day for a week. the reappearance of his previously extinguished fear when alex rode a bike on the same trail two weeks later best illustrates

I believe the answer is: spontaneous recovery

spontaneous recovery refers to the automatic response that we give in a certain situation which created from past conditioning.

In most cases, spontaneous recovery happened after we suddenly re-exposed to a certain stimulus that trigger a huge biological or psychological response in the past