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The `C (gcc)` implies extensions - it will only work with that specific compiler under it's default setting GNU17 (equivalent to `-std=gnu17`), which is mostly a superset of standard C. This also includes a number of POSIX functions.

Another option is standard C, meaning no extensions and no constraint or syntax violations. This would be equivalent to `gcc -std=c17 -pedantic-errors`. As a subset of standard C, there is yet another option: strictly conforming standard C, meaning there can also be no poorly-defined behavior.

The definition of a strictly conforming program can be found in C17 4/5:

> A _strictly conforming program_ shall use only those features of the language and library specified in this International Standard. It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit.

Solutions in standard C and/or strictly conforming should be labelled as such by the poster. Otherwise, the default behavior of the specific compiler with no options passed to it is assumed to apply.

C++ (g++) behaves mostly the same, though in C++ there's also the boost library which has to be included explicitly. I think boost is to be regarded as any 3rd party lib and solutions using it will then be `C++ with boost`.