"mariah is a neuroscience graduate student interested in understanding how the human brain processes early childhood memories. what technique should mariah use to determine which brain regions are activated when someone recalls their first memory? "

I believe the answer is: positron emission tomography

positron emission tomography  refers to a medical technology that is used to provide functional imaging on parts of our body. This technology probably more known as PET-scan. When we use this to provide imaging to human's brain, we can observe several part of the brain would light up during a certain activity due to heat.