I have(Account and opportunity)object there's picklist field in acc. object as rating (Hot,Warm,Cold) make a formula field in opp. name as rating when i choose in acc. rating Hot then it gonna show in opp.rating A.what will be the syntax?

Try this:

IF(
  ISPICKVAL(Account.Rating, "Hot"),
  "A",
  IF(
    ISPICKVAL(Account.Rating, "Warm"),
    "B",
    "C"
  )
)