form the main conducting part of the phloem.
The correct option is D Sieve tubes
Sieve tubes formed of sieve cells form the main conducting part of the phloem. Sieve tubes are suited for translocation of food because they possess broader lumen and perforated cross walls. Companion cells are thin walled elongated parenchymatous cells that help the sieve tubes in the conduction of food material. Phloem fibers are dead sclerenchyma fibers formed of elongated cells. These provide mechanical strength. Phloem parenchyma store food.