a. vessel elements.
b. sieve cells.
c. tracheids.
d. companion cells.
e. cambium cells.

i think that answer should be cambium

bcoz cambium is a tissue type not a cell type...

well im not sure
btw gud question

Sieve Cells and Tracheids are also part of the vascular transport system. Companion cells are part of the phloem although does not conduct. sugars. Vessel elements are like tracheids allowing water to flow through. The vascular cambium adds secondary xylem and phloem and sits in between them both. The bark is classified right down to the vascular cambium (the secondary phloem being on the outside).

Based on that I'd say that they were all part of the vascular system. The cambium cells are apparently part of the ground tissue though.