Which one of the following parts of the pitcher plant becomes modified into a pitcher?

The pitcher in carnivorous plants is a modified leaf. In genera such as Nepenthes and Sarracenia the leaf-lamina is transformed into a pitcher‑like structure (often with a coloured lid and digestive surfaces) while the petiole may become a tendril that supports the pitcher. Standard botanical definitions also describe the pitcher as a specialized leaf in which parts of the blade and petiole form an elongated receptacle for trapping insects, confirming that the organ involved is the leaf rather than stem, stipule or petiole alone. Therefore the correct choice is Leaf (option 2).