Final answer:
In the context provided about the rabbit named Milky, the underlined word describing its gentle nature is best replaced by 'kind'.
Step-by-step explanation:
The underlined word in the description of the rabbit, Milky, can be replaced by 'kind'. This choice is based on the context of the sentence which highlights Milky's gentle nature in never biting or kicking, which is a characteristic of being kind to others. Other options such as strong, smart, brave, and calm, though potentially applicable, do not fit as seamlessly into the sentiment being expressed about Milky's temperament.