Jenny's earliest memory is of her baby brother being born when she was 3 years old. Her inability to remember anything before this is an example of:

Jenny's inability to remember experiences before the age of 3 is an example of infantile amnesia, which is a common phenomenon in child development. Most individuals cannot recall memories from the early years of their life, usually before the age of 3 or 4. This absence of early memories is believed to be linked to the ongoing development of the brain, particularly the hippocampus, which plays a crucial role in forming and storing memories.

During infancy and early childhood, the brain undergoes significant changes in structure and function that affect memory formation. Additionally, the development of language skills and the ability to narrate experiences also contribute to the formation of long-term memories. As children grow older and their cognitive abilities develop further, they gradually begin to form and retain memories more effectively.

The other concepts listed do not accurately describe the situation. Long-term memory refers to the ability to store information over extended periods, while childhood nostalgia pertains to fond recollections of past experiences during childhood, and memory retention is the ability to retain information once it has been learned. None of these capture the specific phenomenon of being unable to recall memories from early life, which is precisely what infantile amnesia illustrates.