Final answer:
Children generally require less protein than adults, so the four-year-old, Christian, needs the least protein out of the given options.
Step-by-step explanation:
The individual who needs the least protein out of the given options is Christian, a 60-pound four-year-old. Children generally require less protein than adults due to their smaller size and lower metabolic needs. Additionally, Christian, being a four-year-old, is still growing and developing, which also influences his protein needs.