Explanation:

Recently, Belgrade (Serbia) has introduced liquid tree, an urban photo-bioreactor, to combat air pollution. • LIQUID 3 contains water and uses microalgae to reduce greenhouse gas emissions and improve air quality by binding carbon dioxide and producing pure oxygen through photosynthesis. • LIQUID 3 is more efficient than trees, replacing two 10- year-old trees or 200 square meters of lawn. • A photobioreactor (PBR) can be defined as an enclosed, illuminated culture vessel designed to control biomass production through adjustment of the operating parameters. • Hence option (a) is the correct answer.