asked to support the claim?
identify it
net CO2 is likely to increase if seasonal snow cover arrives earlier relative to the onset of soil surface freezing
the claim is unnecessarily complicated?
simplify it
if snow comes earlier than freezing, there will be more CO2
the options mention stuff you've never heard about?
read the passage
increased carbon dioxide (CO2) absorption through greater productivity of marsh Labrador tea (R. tomentosum) plants and other vegetation
increased CO2 output through greater heterotrophic respiration