Marla stops outside her 5-year-old son's room where he is playing alone with Legos. She hears him saying, "The green one goes here. I need a blue one to go here. My tower is finished!" Marla's son is demonstrating:

Private speech

Explanation:

Private speech is a term in developmental psychology, that describes the speech spoken to oneself for communication, self-guidance, and self-regulation of behavior.

Often common among children between age two and seven, it is a spontaneous self-directed talk in which a person "thinks aloud," most especially for the purpose of regulating cognitive processes and guiding behavior. While it is audible, it is neither intended for nor directed at others.

Hence, in this case, Marla's son is demonstrating a Private speech.