Answer
b. sieve tubes
Work Step by Step
Phloem is made up of two types of cells: sieve elements and companion cells. When mature, sieve elements lose most of their organelles so they are better able to conduct sugars. The metabolic functions of the sieve elements are carried out by the companion cells, which are connected to the sieve elements through openings called plasmodesmata.
Sieve tubes are created by stacking sieve elements together. Sieve tubes transport sugars from regions where they are made or stored to other areas of the plant where the sugars are either stored or used by other cells.
