Answer:
sixth
Step-by-step explanation:
Joshua's behavior places him in the sixth substage of the sensorimotor stage. This is known as Early Representational Though and occurs in children between the ages of 18 to 24 months. During this time, children begin to understand the world through various mental operations (such as talking, singing, imitating specific behaviors, etc.) as opposed to doing everything purely through actions. Which is what Joshua is demonstrating by not only playing with the train but mimicking the sounds that trains make.