Why Is C Language Is Called "C "? Why Not "D"?

The first version of UNIX was written in the low-level PDP-7 assembler language.Then they developed a High-level language  called 'B' and it got its name  since it was developed based on BPCL language.    Then Dennis Ritchie developed the Language 'C' due to the arrival of PDP-11 computer.They named it as 'C' since the previous language they used was 'B' and they want to be named in sequential order i.e after 'B' , 'C' comes which follows sequential order.Hence they didn't  name it a 'D'.

Languages are usually came from basic combined programming language.
So the first language was named as b-language
and later it was advance and named as c-language

If I am right that you might be wondering about the origin of the C language and where it got its name. Then here is, C was created by Dennis Ritchie at the Bell Telephone Laboratories in 1972. The language was not created for the fun, but for a specific and special purpose that to design the UNIX operating system
The C is such a powerful and flexible language, its use quickly spread beyond Bell Labs. Programmers everywhere began using it to write all sorts of programs. Then, after that the different organizations began utilizing their own versions of C. In response to this problem, the American National Standards Institute (ANSI) formed a committee in 1983 to establish a standard definition of C, which became known as ANSI Standard C. With few exceptions, every modern C compiler has the ability to adhere to this standard.
Now, what about the name? The C language is so named because its predecessor (An activity that must be completed or be partially completed before a specified activity can begin is called a predecessor) was called B. The B language was developed by Ken Thompson of Bell Labs. Now, you should be able to guess why it was called B.

Just before the C Language there was A after which came B..C was developed at AT&T Bell Laboratories

A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that specify the behavior of a machine, to express algorithms precisely, or as a mode of human communication.