Sarah shows her baby a colorful block several times. the baby looks carefully at the block, at first, but then turns her attention to a different toy after seeing the block a few times. the baby is showing

habituation

Habituation is mental learning process in infants whereby repeated exposure to an object or stimulus results in a decrease in response to it. In this case, after Sarah's baby has been exposed to the colorful block for a long duration, she became "habituated" or familiar with it, after which she lost interest in observing in and moved on to another toy.