Start by fixing the meaning of "juvenile" in your mind: it is used for young people or for behaviour that seems immature for one's age, such as "juvenile jokes" or "juvenile courts" dealing with young offenders.
Now scan the options for a word carrying that same young or childish flavour. "Rustic" points to village life, "rude" points to bad manners, and "snug" points to comfort, so none of these share the idea of youth or immaturity.
"Infantile" is built from "infant", a baby, and it directly describes something childish or immature, which mirrors "juvenile" closely.
So the closest synonym for "juvenile" is option D, "Infantile".