When I learnt programming (back in early 2000s) the textbook said C is a high level 3rd generation language with 4th gen languages being something higher (I don’t remember what examples were given specifically). This is back when the java applets and action script for flash were the hot things. How I miss the days without the world being cursed by JS.
I think C was 2nd, 3. is Java and Python, 4 SQL and 5th would be some hypothetical AI instruction language?
1st level is direct binary code as was done with punch cards. Assembly language is a 2nd level language. C is a level above, thus it’s level 3.
When the gp’s book says that C is a third generation language: I would guess the first generation is Fortran and the second generation contains ALGOL and BCPL. C was heavily influenced by BCPL. (get it? C comes after B)