The 'Common Carbon Metric', supported by UNEP, has been developed for

(a) assessing the carbon footprint of building operations around the world ✓ UPSC's answer
(b) enabling commercial farming entities around the world to enter carbon emission trading
(c) enabling governments to assess the overall carbon footprint caused by their countries
(d) assessing the overall carbon footprint caused by the use of fossil fuels by the world in a unit time

Why the answer is (a)

• The Common Carbon Metric was developed by UNEP's Sustainable Buildings and Climate Initiative (UNEP-SBCI) with the World Green Building Council.

• It provides a standard protocol to measure energy use and greenhouse-gas emissions from the operation of buildings (kg CO2/m²/year and per occupant), enabling comparison across countries — option (a).

• Buildings account for roughly a third of global energy-related emissions, hence the need for a common measure.

• It is not a farming carbon-trading tool (b), a national inventory method (c) or a global fossil-fuel metric (d).

• Hence option (a).

Why the other options are wrong

(b) enabling commercial farming entities around the world to enter carbon emission trading: It is not a farm carbon-trading mechanism.
(c) enabling governments to assess the overall carbon footprint caused by their countries: It measures buildings, not national totals.
(d) assessing the overall carbon footprint caused by the use of fossil fuels by the world in…: It is not a global fossil-fuel emissions metric.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2021, held on 10 October 2021. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.