Option 1: Presence of `eye` on potato is a feature that indicates the modification of the stem of a plant. The `eye` on a potato is actually a bud that has the potential to grow into a new plant. It is a specialized structure on the stem of the potato where multiple buds are present. These buds are responsible for producing new shoots and roots, allowing the potato to sprout and develop into a new plant.

Option 2: `Scale` found in onion is actually a modified leaf that protects the inner layers of the bulb. While it is a modification, it is not specific to the stem of the plant, as scales can also be found on the leaves of other plants.

Option 3: `Tendril` found in pea is a modified leaf or stem that helps the plant in climbing or supporting itself. Tendrils are not a modification of the stem alone, as they can also be found on leaves or inflorescences.

Option 4: `Hair` present in carrots is not a modification of the stem. Carrot hairs are actually root hairs that assist in the absorption of water and nutrients from the soil.

Therefore, the correct answer is Option 1, and the indication for modification of the stem of a