Anomalous pair among the following is

In Mendeleev's Periodic table, at few places the elements are arranged in the reverse order of their atomic masses.

Hence The pair of the element in the periodic table where the element occurring before has more atomic mass than the later occurring one is known as 'Anomalous pair'.

These are $\mathrm{Ar}-\mathrm{K}, \mathrm{Co}-\mathrm{Ni}, \mathrm{Te}-\mathrm{I}, \mathrm{Th}-\mathrm{Pa}.$