If you're asking whether it's possible for an automated decompiler to produce C code as output for a given C++ input binary (rather than producing C++ code as output), the answer is that most decompilers are going to do this anyway, and are not going to give you a choice as to which language they produce as output. Machine-code decompilers generally produce a C-like pseudocode, which, in general, cannot be immediately recompiled to C code. A better question would be "to what extent can the C-like pseudocode produced by decompilers represent features of C++ that are not in C", to which the answer depends on the specific decompiler in question.
If you're asking whether it's possible to manually decompile a C++ program into a C program, the answer is yes. Just as you can take a program written in C++ and manually translate it into C, you can manually decompile a binary into any language you want. There's going to be a lot of manual work regardless of which language you choose to decompile the program into. Although C and C++ are not the same language, the first C++ compilers worked by translating C++ code into C code, and using a regular C compiler on the result (see: CFront https://en.wikipedia.org/wiki/Cfront). You can simulate most C++ constructs in C, although it's going to be a lot more work than simply taking advantage of the features introduced by C++.