why c method using in win32
I don't really understand your question. C is a language. It also happens to be the language Windows is (mostly) written in and the native language of the Win32 API but many languages are capable of calling C APIs and using libraries written in C.
Win32 was derived from the Win16 API.
When the Win16 API was developed there were two main programming languages for Windows: C, and assembly.
Win32 provides a set of APIS which can be called by a program written in C. C FYI is a programming language.
If you mean why Win32 API is composed by C functions, the reason is that the Windows operating system itself is written in C.
I read this as: "Why does the Win32 API use C?"
And what's wrong with their choice of C? :-)
The Windows API dates back a long way, well before the advent of 32-bit programming on PCs. At that time (1983-ish), C was the most widely used systems language, and enjoyed strong compiler support on PCs. C++ was still in its infancy. Those factors probably made the initial choice of C a bit of a no-brainer.
As to why it's stayed in C, C is still a high-performance and very effective systems programming language, and Microsoft has little incentive to toss the massive investment they have in that C codebase. Plus, Microsoft has been careful to maintain backwards compatibility for users, which means preserving not just the API but the language the API is written in.
There's a good reason why any operating system's API should be (and almost always is) written in C or a similarly low-level language: It's easy for high-level languages to wrap and use an API from a lower-level language, but it's difficult or impossible to do the reverse without incurring so much overhead that the whole usefulness of the low-level language is lost.