Option 1: Childish -> Puerile means immature or childishly silly, making this a direct synonym.
Option 2: Melancholy -> Means sad or gloomy, which is unrelated to the meaning of puerile.
Option 3: Cosmopolitan -> Means sophisticated or worldly, which is actually opposite to puerile.
Option 4: Precocious -> Means advanced or mature for one's age, which contradicts the immature nature of puerile.
Hence, Option 1: Childish -> "Puerile" derives from the Latin word "puer" (meaning boy/child) and describes something that is childishly silly, immature, or trivial. In the passage, the narrator dismisses his thought as "puerile" because it's an unrealistic, naive fantasy about what he could have achieved. -> correct