ClO⁻₄<ClO⁻₃<ClO⁻₂<ClO⁻
ClO⁻₄>ClO⁻₃>ClO⁻₂>ClO⁻
ClO⁻₃<ClO⁻₄<ClO⁻₂<ClO⁻
ClO⁻>ClO⁻₃>ClO⁻₂>ClO⁻₄
The correct option is ClO⁻₄<ClO⁻₃<ClO⁻₂<ClO⁻. ClO⁻₄<ClO⁻₃<ClO⁻₂<ClO⁻ is the correct increasing order of Bronsted base strength. With an increase in the number of oxygen atoms, the extent of delocalisation of negative charge by pπ-dπ bonding increases. This stabilizes the ion and decreases the tendency to accept a proton. This decreases the basicity.