Final answer:
Merlin advised Uther to hide his son after birth to protect him from Uther's enemies who would seek revenge by killing the child.
Step-by-step explanation:
In the text "King Arthur and the Sword in the Stone," Merlin tell Uther that his son would have to be hidden after birth because of the danger that awaited him if his identity was known. The correct answer is Option 2: Merlin feared that Uther's enemies would kill the boy to get revenge on Uther. This protective measure was a common trope in tales of ancient times, where children of great importance were often hidden away to safeguard them against political enemies or threats that sought to disrupt royal lineage or seek vengeance.