Explanation: In SITOR (NBDP) operations, the specific characters transmitted to terminate a direct TELEX connection are "KKKK". This sequence is a standardized procedural signal within the SITOR protocol. It serves as an unambiguous command to the receiving station and the automated system to indicate that the transmission is complete and the communication link should be disconnected. Other options are incorrect because they are not recognized as termination signals within the SITOR protocol. "...." is not a command, "END+" is not a standard SITOR sequence, and while "EOM" (End of Message) signifies the content's conclusion, "KKKK" is the specific instruction to actually tear down the technical connection. These defined procedural signals ensure reliable and orderly management of the radio link.