At least ________ of the carbon dioxide fixation on earth is carried out by algae through photosynthesis.

a half

The correct answer is Option (2) – a half

Out of the choices you listed, at least a half of the carbon dioxide fixation on Earth is carried out by algae through photosynthesis.

Algae, particularly phytoplankton in the oceans, are major contributors to the global carbon cycle. Estimates suggest they account for around 50% of the total carbon dioxide fixation on Earth.

Algae are a diverse group of predominantly aquatic photosynthetic organisms, including cyanobacteria, green algae and other eukaryotic algae. They account for more than 50% of the photosynthesis that takes place on Earth. Photosynthetic efficiency is generally higher in algae than in higher plants, because of a wide range of antenna pigments to harvest more solar energy and a variety of carbon dioxide-concentrating systems to increase carbon dioxide (CO₂) concentration around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).