An ECO-MARK is a label or certification that is used to identify an environmentally friendly consumer product. This means that option 2, which states that an ECO-MARK is a scheme for labelling environment friendly consumer products, is the correct answer.

Option 1, which suggests that an ECO-MARK is for labelling pollution-free industrial units, is incorrect. This is because an ECO-MARK is specifically used for consumer products, not for industrial units.

Option 3, which claims that an ECO-MARK is a cost-effective production technique, is also incorrect. An ECO-MARK is not related to a production technique but rather to the environmental impact and sustainability of a product.

Option 4 proposes that an ECO-MARK is an international certification recognizing eco-friendly buildings. While this seems related to sustainability, it is not the correct answer. An ECO-MARK specifically refers to consumer products and not building certifications.

Therefore, the correct answer to the question is option 2, as it accurately describes an ECO-MARK as a scheme for labelling environment friendly consumer products.