Final answer:
The medical assistant should place the tape measure across the forehead and the occipital prominence to measure the circumference of an infant's head.
Step-by-step explanation:
To measure the circumference of an infant's head, the medical assistant should place a tape measure across the forehead and the occipital prominence. This is typically done just above the eyebrows and over the ears, ensuring that the tape measure is snug but not too tight. This measurement helps to track the growth and development of the infant's head.