When samer was born, stimulation in his brain resulted in a massive overabundance of synapses. neurons that were seldom stimulated soon lost their synapses. this process is known as?

Synaptic pruning.
Synaptic pruning is the process of eliminating neurons and synapses that are not profitable for the well-function and efficiency of neuronal transmissions. Since the beginning of the brain development until the baby is born, and after, up to 2 years of age, new neurons and synapses are created in an exponential way. This pruning starts early from 2 years of old until 10 years of old or later.