Suppose that the udp receiver computes the internet checksum for the received udp segment and finds that it matches the value carried in the checksum field. can the receiver be absolutely certain that no bit errors have occurred? explain

It does not appear that the receiver would be absolutely certain that no bit errors have  occurred. As long as the checksum and its 1s complement equal the value that was originally  sent, it will verify. Basically, a 0 or 1 can possibly be swapped somewhere in the checksum,  but no bit errors will be detected as long as the addition sent by the sender and checksum  received by the receiver match. No, the receiver cannot be absolutely certain that no bit errors have occurred. This is because of the manner in which the checksum for the packet is calculated.