The key to solving this question is to track the transfers of each individual: Saxena, David, Jain, and Kumar.
In 1970, they were at places P, Q, R and S, but we don`t know who was where.
In the first transfer (1972), Saxena and Jain switched places and Kumar and David also switched spots. The specific places don`t matter as much as understanding that each individual moved to a new location.
In the second transfer (1973), David and Jain interchanged places, and Saxena and Kumar interchanged places as well.
So, to have all persons posted at all four places, we need to ensure each person moves to a new spot they have not previously occupied.
Option 1 and 2 aren`t correct because they both suggest repeating changes made in 1972 and 1973.
Option 4 is incorrect as the required condition can be met.
Option 3 suggests a new interchange. David and Kumar, as well as Saxena and Jain, haven`t interchanged places in the previous transfers. Therefore, the correct answer is 3.