(i) Carbon offsets are credits for reductions in greenhouse emissions: This is correct. An offset represents a reduction in GHG emissions or an increase in carbon storage (e.g., through afforestation) that is used to compensate for emissions occurring elsewhere.
(ii) Carbon offsets are tradable unused quota of GHG emissions permitted to the countries: This describes carbon credits (or allowances) under a cap-and-trade system, specifically related to national commitments (like under the Kyoto Protocol's Assigned Amount Units - AAUs). While carbon offsets can be traded, the fundamental definition of an offset is a reduction achieved, not an unused quota. So, this statement is more accurately defining a cap-and-trade allowance, not an offset. Therefore, this is incorrect as a primary definition of offsets.
(iii) Carbon offsets are quantified and sold in metric tons of carbon dioxide equivalent: This is correct. Carbon offsets are measured and traded in units of one metric ton of carbon dioxide equivalent (CO2e), reflecting the global warming potential of various GHGs.
(iv) Carbon offsets are total emissions of GHGs over a period of time by an individual: This describes an individual's carbon footprint, not a carbon offset. So, this is incorrect.
Based on the analysis, only (i) and (iii) are correct definitions/characteristics of carbon offsets.
