"Juvenile" describes something or someone that is young, immature, or childish in nature. We need the option closest to this sense of youthfulness or immaturity.
- Option A, "Rustic": This word means related to the countryside or rural life, which has no link to being young or childish.
- Option B, "Rude": This means impolite or disrespectful, a completely different quality from being youthful.
- Option C, "Snug": This means cosy or comfortably fitting, which again has no connection to age or immaturity.
- Option D, "Infantile": This word means babyish or childishly immature, which is exactly the sense carried by "juvenile".
The correct choice is option D, "Infantile".