Which of the following is a type of multicellular algae?

Kelp represents a type of multicellular algae, specifically belonging to the brown algae group (Phaeophyceae). Kelp is characterized by its large size and complex structure, often forming underwater forests that provide habitat and food for various marine organisms. In addition to being multicellular, kelp has differentiated tissues and organs similar to those found in terrestrial plants, which allows it to perform essential functions such as photosynthesis and nutrient absorption efficiently.

This unique structure and growth habit make kelp an essential contributor to marine ecosystems, as well as a significant resource for various human uses, including food, fertilizers, and industrial applications. The other options, while they may involve algae or algal-like organisms, do not fit the criteria of multicellular algae to the same extent. For instance, diatoms are unicellular, Volvox is a colonial green alga that can be considered a simple multicellular organism but does not exhibit the complex structures found in kelp, and Chlorophyta refers broadly to green algae, which includes both unicellular and multicellular species but does not specifically name a multicellular representative.