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

assessing the carbon footprint of building operations around the world
enabling commercial farming entities around the world to enter carbon emission trading
enabling governments to assess the overall carbon footprint caused by their countries
assessing the overall carbon foot-print caused by the use of fossil fuels by the world in a unit time

Answer

assessing the carbon footprint of building operations around the world

(a) Correct. The Common Carbon Metric (CCM), supported by UNEP-SBCI, was developed as a common method for measuring and reporting energy use and greenhouse gas emissions from building operations globally.
(b) Incorrect. It is not a tool for bringing commercial farming entities into carbon trading; it is focused on the buildings sector.
(c) Incorrect. It does not measure the total national carbon footprint of countries; its scope is building operations, not whole economies.
(d) Incorrect. It is not a metric for global fossil-fuel emissions per unit time; it specifically concerns carbon emissions from buildings in operation.