Introduction:
Algae are a diverse group of photosynthetic organisms that can be found in various aquatic environments. They play a crucial role in the production of oxygen and serve as a food source for other organisms. Some classes of algae are known for their commercial importance due to the production of hydrocolloids such as algin and carrageenan.

Explanation:
The correct answer is option 'D' - Both B and C (Rhodophyceae and Phaeophyceae). Let's understand why:

Chlorophyceae (Green algae):
- Chlorophyceae is a class of green algae that includes various species found in marine, freshwater, and terrestrial habitats.
- While some green algae species produce hydrocolloids, they are not commercially significant compared to the other classes mentioned in the options.

Rhodophyceae (Red algae):
- Rhodophyceae is a class of red algae known for their red pigmentation due to the presence of phycoerythrin.
- Red algae are commercially important for their production of hydrocolloids like carrageenan. Carrageenan is widely used in the food industry as a gelling, thickening, and stabilizing agent.

Phaeophyceae (Brown algae):
- Phaeophyceae is a class of brown algae commonly found in marine environments, particularly in cold waters.
- Brown algae are also known for their production of hydrocolloids, primarily algin. Algin is extracted from brown algae and is used in a wide range of industrial applications, including food, pharmaceuticals, and cosmetics.

Conclusion:
In conclusion, the class of algae known for producing hydrocolloids such as algin and carrageenan, which are used commercially, includes both Rhodophyceae (red algae) and Phaeophyceae (brown algae). These hydrocolloids have various industrial applications, making these algae classes economically significant.