Correct Answer - D
`pH=pK_(a)+log``([HCO_(3)^(-)])/([H_(2)CO_(3)])`
`implies 7=7-log4+log``([HCO_(3)^(-)])/([H_(2)CO_(3)])implies ([HCO_(3)^(-)])/([H_(2)CO_(3)])=4`
`%` of carbon in the form of `HCO_(3)^(-)`
`=([HCO_(3)^(-)])/([HCO_(3)^(-)]+[H_(2)CO_(3)])xx100=(4)/(1+4)xx100=80%`