The correct option is D Cambium
Monocot plants cannot be grafted as they lack the cambium tissue, which is present in dicot plants. Cambium tissue is a meristematic tissue, which is present in the vascular bundles of the dicot plants. This tissue has a regeneration capacity due to which it can form new tissue at the time of grafting. The lack of cambium in monocots makes it impossible to graft. Thus, the correct answer is option A.