Final answer:
The child's understanding that his mother exists while she is not present indicates he has reached the stage of object constancy in Mahler's separation-individuation process.
Step-by-step explanation:
The child's awareness that his mother still exists even though she is not presently visible in the room illustrates the concept of object constancy. According to Margaret Mahler's stages of separation-individuation, the ability to maintain an image of an absent caregiver and comprehend that they still exist is attained in the stage of object constancy. This stage is the culmination of the separation-individuation process and typically develops in children between the ages of 3 to 5 years old. The presence of object constancy indicates that the child has overcome separation anxiety and has acquired a stable sense of the caregiver's presence, allowing for emotional attachment even when they are not physically together.