[__d]

You need to understand the timescales of C.

C89 (aka ANSI C) was "standard" C for years. C99 was released, and largely ignored until the mid-2000's, and then gradually began to grow. As of about 2015, C99 was probably the "standard" C dialect, but there is still a lot of C89 out there, especially where you have niche toolchains.

I wouldn't seriously consider C11 unless you're certain that the toolchain supports it, and there's some feature that you really need. And that's pretty unlikely.