Function of companion cells is Providing energy to sieve elements for active transport. Providing water to phloem. Loading of sucrose into sieve elements. Loading of sucrose into sieve elements by passive transport.

D. Loading of sucrose into sieve elements
Reason: Companion cells move sugar and amino acids into and out of the sieve elements. In tissues such as leaf companion cells use transmembrane proteins to take up sugar like Sucrose and amino acids by active transport. Movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a seive tube. Loading sets up a water potential gradient that facilitates movement of sugar.