GKFix MCQ Question

For a salt MX where X⁻ is a conjugate base of a weak acid, how is the fraction 'f' defined?

• A. f = [HX]/[X⁻]
• B. f = [H⁺]/Ka
• C. f = [M⁺]/[X⁻]
• D. f = [X⁻]/([X⁻] + [HX])