Seaweed and kelp are BROWN algae. Option A is correct.
A kelp forest is conformed by a large grouping of kelp (large brown algae - seaweed found in cool, relatively shallow waters close to the seashore). Kelp forests can provide food, shelter, and protection from predators for a wide range of sea creatures.
Seaweed or macroalgae refers to several species of macroscopic, multicellular, marine algae. It includes some types of red, brown, and green macroalgae.