When we are executing dd command, which write function gets called. As per my understanding, dd command is not filesystem specific, so no file system's file_operations is involved. Please correct If I am wrong here.
I would like to know which file_operations is involved in carrying out dd operation?
That depends on what you write to.
Either it is a regular file and file system specific calls are used or it is a device and you eventually use to the target disk (or whatever) underlying driver.
http://www.makelinux.net/books/ulk3/understandlk-CHP-14-SECT-5#understandlk-CHP-14-SECT-5