embedded system software

The definition of embedded software has expanded somewhat to include development on platforms that use different versions of UNIX already ported and given to developers as a part of the development package.

If that is the case you most likely will use GNU C/C++. However, I don't consider this an embedded system.

Plus you have Android, plus you get Windows CE so the boundaries of what is "embedded" are blurred somewhat (especially in job ads :))

Having stated the above, it is still the case that 90% or more of classic, bare-bone embedded system software projects are written in C.

The reasons why this has occurred:
C++ has a heavyweight run-time environment and porting to a new platform would require a fair bit of work before you even start coding the application (examples - exception processing);
C++ cross compilers generally are less mature. Having compiler problem is the last thing you want in an embedded project so choose wisely;
Good C cross-compilers are extremely mature and generate phenomenally efficient code (important for real-time environment);
Embedded C++ (EC++) has not taken off and will not in my opinion;
Scale of the project is typically such that C++ advantages are not as obvious as for desktop or mobile app development.

Overall, stick with 'C' you cannot go wrong.

Optimality will depend on your context of what you are trying to do and the kind of hardware you will be running on, if you are familiar with c or c++ then probably it will be better and more performant.