The correct answer is option 3: Bio-bricks.

Bio-bricks are blocks that are created using organic materials, such as bacteria or algae, mixed with sand. These materials are often waste products or renewable resources, making bio-bricks an eco-friendly alternative to traditional clay or concrete bricks. Bio-bricks are created through a process called microbial-induced calcite precipitation, where bacteria or algae are used to bind the sand particles together, forming a solid block.

Option 1, Green cover, refers to the vegetation or plants covering an area. Option 2, Green Ministry, likely refers to a government department or organization focused on environmental issues. Option 4, Pro-biotic curd, is a type of yogurt that contains live bacteria cultures which are believed to have health benefits.

In summary, green blocks are bio-bricks made from organic materials and sand.