The correct answer is Option 2: Leaf. In pitcher plants, the leaf is the part that becomes modified into a pitcher.

Pitcher plants are carnivorous plants that have evolved specialized leaves to trap and digest insects and other small prey. These modified leaves form a pitcher-like structure that contains a pool of digestive enzymes or fluid, which helps the plant to obtain nutrients in nutrient-poor environments.

The modified leaf of a pitcher plant typically has a tubular or elongated shape, resembling a pitcher or a jug. It may have a lid or a hood called the operculum, which helps to prevent rainwater from diluting the digestive fluid and also acts as a barrier to keep prey from escaping. The rim of the pitcher often has nectar glands or attractive patterns to lure insects inside.

Once an insect or small organism is lured into the pitcher, it encounters downward-pointing hairs or slippery surfaces on the inner walls of the pitcher that make it difficult to climb out. The prey falls into the digestive fluid at the bottom of the pitcher, where it gets trapped and is gradually broken down by the enzymes secreted by the plant. The dissolved nutrients are then absorbed by the plant to supplement its diet in nutrient-deficient habitats.

It`s important to note that while the stem (Option 1), stipule (Option 3), and petiole (Option 4) are all parts of a plant, they are not typically modified into pitchers in pitcher plants. The leaves undergo significant modifications to form the specialized pitcher structures, while the stem, stipule, and petiole serve different functions within the plant, such as providing support, attachment, or protection.