Heterocysts, specialized for nitrogen fixation, occur in certain
A. Red algae (Batrochospemmum)
B. Green algae (Spirogyra)
C. Blue-green algae (Anabaena)
D. Brown algae(Laminaria)

Hint:-Algae classified on the basis of the pigments found in them and the reserve food material. Heterocysts are the specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, during nitrogen starvation. Example: Nostoc and Anabaena.

Complete step-by-step solution:-
They fix nitrogen from dinitrogen which is present in the air using nitrogenase.
Nitrogenase gets inactivated in presence of oxygen, so the heterocyst creates an anaerobic environment to produce three additional cell walls, which form a hydrophobic barrier for oxygen, degrade photosystem II, to produce oxygen, and up-regulate glycolytic enzymes.
It also produces proteins that scavenge the remnant oxygen and contain polar plugs composed to reduce the process of diffusion.
Cyanobacteria obtain carbohydrates by photosynthesis and the lack of the water-splitting in photosystem II prevents heterocysts from performing photosynthesis. The vegetative cells provide it with carbohydrates in the form of sucrose. The carbohydrate and fixed nitrogen are exchanged through cell channels present in the filament. Heterocysts also maintain photosystem I, by aiding them in generating Adenosine Triphosphate by the process of cyclic Photophosphorylation.
Red algae are called Rhodophyta and are among the oldest eukaryotic algae.
The green algae are a large grouping of algae and are called Chlorophyta.
The brown algae are multicellular algae, including many seaweeds found in the colder waters in the Northern Hemisphere.
So, option (D), Brown algae(Laminaria) is the correct option.

Note:- Anabaena cyanobacteria and Azolla plants form a Symbiotic relationship where Anabaena lives on the stems and leaves of Azolla plants. The Azolla plant performs photosynthesis and provides Anabaena food and nourishment. While the heterocysts provide the Azolla plant with ammonia which is required for growth.