Symbolically, the time reversal test is satisfied if :

The correct answer is Option (4) → $P_{01}×P_{10}=1$

The five reversal test for an index number requires that if you reverse base and current periods, the product of 2 index numbers should be 1.

Considering $P_{01}$ is the index from period 0 to period 1 and $P_{10}$ is the index from period 1 to period 0, then the time reversal test is satisfied if $P_{01}×P_{10}=1$.