i have been allocated any A college and i choose float option and in 2 round of counselling i got B college then which college should i got

After you choose float option , it means in case of upgradation you will be upgarded to the better option as per your choice filling. Thus you will be allotes the upgraded seat(B) thus making the last seat (A) Vacant for other candidates lower in the list. So your seat A will be given to other student and you will get seat B.

In short , you will loose A and get B