Explanation:

Pascal is a well-known entity in computer science. **Pascal (a) is a computer language**; specifically, it is a high-level, imperative, and procedural programming language designed by Niklaus Wirth in the late 1960s and published in 1970. It was primarily created as a teaching tool for structured programming concepts. **(b) is a unit of computer** is incorrect; Pascal is software, not a hardware component or a measurable unit. **(c) is a computer operating system** is also incorrect; an operating system (like Windows or Linux) manages computer hardware and software resources, whereas Pascal is a language used to write programs that run on an operating system. **(d) is a type of computer** is incorrect; types of computers refer to hardware categories (e.g., personal computer, mainframe), not software. Therefore, the only accurate description among the choices is that Pascal is a computer language.