Final answer:
Cells responsible for bulk flow are tracheids, vessels, and sieve tubes, which facilitate the long-distance movement of water and organic substances in plants through the xylem and phloem. The correct option is a.
Step-by-step explanation:
The cells involved with bulk flow include tracheids, vessels (vessel elements or vessel tube members), and sieve tubes (sieve-tube elements). Bulk flow is responsible for the long-distance movement of fluids in plants, occurring in both the xylem and phloem. Tracheids and vessels are involved in transporting water and minerals in the xylem, while sieve tubes are utilized for transporting organic substances like sugars and amino acids in the phloem. Tracheids and vessels work together to create a tension difference that pulls water up through the plants, aided by water's adhesion and cohesion as well as transpiration. Sieve tubes, composed of sieve-tube elements, allow for the flow of phloem sap between cells through plasmodesmata and perforated sieve plates, while companion cells provide metabolic support.