Piaget's nursemaid told his family that she saved his life from an attempted kidnapping. Piaget believed the nursemaid and later recalled details of the event, even though it never actually took place. Piaget's ability to recall an event that never took place is demonstrative of:

False memory.

Explanation:

A false memory is a memory or recollection of an event that may seem real to a person but is not. It is a fabricated event that is told to a person and made to seem like a real event.

Piaget's nursemaid's insistence that she saved his life is a false memory. And in that method, her version of events seems ingrained in Piaget, which made him remember as if it is real. This is why it is termed a "false memory".

because the early people found out about it.