Hint: Multicellular organisms have different types of tissues, similarly plants also have different types of tissue, where the cell is the structural and functional unit for a tissue or organ or for an organism, based on their function each cell has different structures, even though initially formed cells have all features, but when they grow the features may get modified.
> Option A: Vessels of xylem: Xylem is the conductive tissue found in the plants, which mainly conducts the water and minerals from the roots to all other plant structure, where it contains 4 types of cell structures they contain tracheid’s, and vessel elements and also xylem Parenchyma and fibers, when plants mature and get older, this xylem vessels are dead and lack protoplast and mainly provide mechanical strength, so this is the wrong answer.
> Option B: The parenchyma of pith: Pith is the thing mainly present in the medulla of the stem, where the parenchyma is thin-walled cells, which are useful for storage of food, and these cells are the main fillers of the stem and this cells are alive even after the maturation of tissue, and the cell contains the nucleus, so it is the wrong answer.
> Option C: Cambium of roots: Cambium of the root is the lateral meristem which is useful for an increase in the size of the roots, as we know that meristematic cells are actively dividing cells, so they contain a nucleus, so this is the wrong answer.
> Option D: Sieve tubes of Phloem: Phloem is the conductive tissue, which is useful for the transport of food in the plants, the cells are sieve cell, sieve tubes, and the companion cells, where they transport food by this tube containing the sieves, but this tissues lack or divide of the nucleus, and companion cells are supportive cells of the sieve tubes.
Hence, the correct answer is option (D).
Note: Sieve tubes are the element of the Phloem, where this is elongated living cells, and they have dignified walls, initially these cells have nuclei but later, they have been fragmented and got disappeared, and these tubes have sieves on their transverse end wall useful for the conduction of food.