There are two register R1 and R2. How is it possible to exchange the content of R1 and R2 in a single clock pulse using Common bus architecture?
You can't. CBA doesn't work that way.
Exchanging registers in a single cycle is only possible if both registers can present themselves as input to the other register at the same time. Only a single register can emit to the bus at any time, therefore a minimum of 3 cycles would be required (store R1, move R2 to R1, load R2; or the XOR trick, which still requires 3 operations).