Which phylum contains the largest multicellular forms of algae? In what way are these algae commercially important? Please help me :,)

Phylum Rhodophyta and phylum Ochrophyta.

Explanation:

Phylum Rhodophyta and phylum Ochrophyta are the phylum that contains the largest multicellular forms of algae.  Algae belongs to phylum Rhodophyta also called red algae are used for commercial production of food and gels while on the other hand, algae of phylum Ochrophyta also known as brown algae are used to produce alginates, which are used as food additives in different products and as a thickening material.