Ctrl-C is the appropriate keys to terminate an application running in the terminal foreground. It sends a SIGTERM to the task running in the foreground. When an application receives a SIGTERM it should clean up after itself and then cleanly quit.
Ctrl-Z will send a SIGTSTP. This signal is particularly useful because it doesn't end the execution of the program; it essentially pauses it. You can use this signal along with the
bg commands to move running programs to the foreground or background, executing multiple programs at once, or just to relieve strain when the computer is doing too many things at once and you don't want to kill any running applications.
Ctrl-D does not actually send a signal, but it is used to indicate end of transmission or file. Programs that read standard input may interpret this as a close condition. (For example, it will cause shells on UNIX-like systems to exit.)