The correct answer is option 'A', Chlorophyceae.

Chlorophyceae is a class of algae commonly known as green algae. They are a diverse group of photosynthetic organisms that are found in various habitats such as freshwater, marine environments, and even on land. Green algae are responsible for a significant portion of the total carbon dioxide fixation on Earth through photosynthesis.

Photosynthesis is the process by which green algae and other photosynthetic organisms convert sunlight, carbon dioxide, and water into glucose and oxygen. This process is vital for sustaining life on Earth as it produces oxygen and helps regulate atmospheric carbon dioxide levels.

Why Chlorophyceae?

1. Abundance and Distribution:
- Chlorophyceae is one of the largest and most diverse classes of algae, with over 7,000 identified species.
- They are found in various environments including freshwater, oceans, soil, and even in symbiotic relationships with other organisms.
- The wide distribution and abundance of Chlorophyceae contribute to their significant role in carbon dioxide fixation.

2. Photosynthetic Pigments:
- Chlorophyceae contain chlorophyll a and chlorophyll b as their primary photosynthetic pigments.
- These pigments enable them to capture light energy from the sun and convert it into chemical energy through photosynthesis.
- Chlorophyll a and b have a high affinity for blue and red light, allowing green algae to efficiently utilize the available solar radiation.

3. Carbon Dioxide Fixation:
- Green algae, including Chlorophyceae, are capable of fixing carbon dioxide through the Calvin cycle, a biochemical pathway that converts carbon dioxide into glucose.
- The Calvin cycle is the most common pathway for carbon dioxide fixation in photosynthetic organisms.
- Chlorophyceae contribute significantly to carbon dioxide fixation, playing a crucial role in regulating atmospheric carbon dioxide levels and maintaining the balance of greenhouse gases.

Conclusion:
Chlorophyceae, or green algae, are primarily responsible for at least half of the total carbon dioxide fixation on Earth through photosynthesis. Their abundance, distribution, photosynthetic pigments, and ability to fix carbon dioxide make them a crucial group of organisms in the global carbon cycle.