Hint: Phloem consists of four components namely the Sieve elements, Phloem parenchyma, Companion cells, Phloem fibres and sclereids. Among these, one of the components of phloem develops a secondary cell wall as they grow due to which they are not capable of expanding further. They become dead when attaining maturity. At this stage they also lack protoplast. The dead cells of phloem provide mechanical strength and support to the plants.
Complete answer: The sieve elements of phloem are made up of elongated narrow cells which are connected together to form a food transport network. They lack prominent nucleus and other cell organelles thus completely depending on companion cells for their growth Their main function is to transport food and nutrients to all the parts of the plant. Companion cells are closely associated with sieve elements and provide metabolic molecules for their survival. Phloem parenchyma is mainly responsible for storage of starch, amino acids and proteins. Phloem sclerenchyma is of two forms- Phloem fibres and Sclereids. Phloem fibres are narrow elongated cells whereas sclereids are short irregular cells. The main function is to provide internal strength and support to plants. They both have a thick secondary cell wall made up of a high amount of lignin. Cells having secondary cell walls are incapable of expanding and dividing therefore upon maturity they are dead. Hence Phloem fibres and Sclereids are both dead cells of phloem.
Therefore, Phloem fibres or sclereids are correct answer
Note: Secondary cell wall is found between primary cell wall and plasma membrane. They are usually formed with the help of a primary cell wall. The main function of the secondary cell wall is to provide rigidity and support to the plant. Presence of a secondary cell wall indicates that the cell has stopped growing and therefore will be dead once matured.