Grafting is not possible in

Correct option is C

Grafting is a vegetative method of plant propagation that strictly requires the presence of a continuous vascular cambium layer for successful union between stock and scion. Onion belongs to the group of monocotyledonous plants, in which vascular bundles are scattered and a continuous cambium layer is absent. Due to the absence of cambium, the vascular connection between stock and scion cannot be established, making grafting unsuccessful. Therefore, grafting is not possible in onion under normal horticultural practices.
Information booster: Grafting is widely practiced in dicotyledonous crops such as mango, citrus, brinjal, tomato, and other horticultural plants. Successful grafting depends on proper alignment of vascular tissues, especially cambium, between stock and scion. Monocot plants generally do not respond to grafting due to lack of secondary growth and cambial continuity. Grafting is used to improve disease resistance, plant vigor, tolerance to stress, and yield performance.
Why other options are incorrect: (a) Tomato is a dicotyledonous plant where grafting is commonly practiced for disease resistance. (b) Brinjal is frequently grafted onto resistant rootstocks to manage soil-borne diseases. (d) Potato is a dicot crop and grafting is biologically possible in experimental conditions.