That depends on your definition of common.
Correction to another post: Microsoft did NOT develop C++. Microsoft developed Microsoft Visual C++ which is a Compiler, they did not design the language or ratify any of its standards.
C/C++ today has the most use and is one of the lowest High Level languages you can use, in that it is High Level but can access features on lower levels including direct memory access and native assembly code.
C was the original Language and was essentially Procedural. C++ was the major revision of the language to include Object Orientated Paradigms.
C/C++ are still very 'common' in terms of who uses them and what for. The majority of Modern Operating Systems are still written in C. C++ is therefore the natural accompaniment if you need to focus on performance and efficiency.
Java is another 'common' language this time in terms of its popularity and what it can do. The language is Object Oriented like C++ but Java is significantly simplified and safer to use as many of the dangerous features of C++ are managed by the Java Virtual Machine. Java is also portable and in theory can run on any machine or device that has a Java Virtual Machine. Java is used in enterprise and in commercial use as well as being used by amateur coders alike.
C# is the final major language worth noting for its similarity to Java. C# was developed by Microsoft and aims to take C++ and add many of the feature that were added to Java but in a manner that fitted Microsoft's needs. C# is therefore common for Windows Games and for Windows enabled devices.
Which language you choose will depend on what you want to ultimately do. There are many others most notably Python deserves a mention but we begin venturing into the world of Scripting vs Programming and we diverge into a completely separate debate.
The easiest of all the languages named to use and learn would be Java. Java's documentation is extensive in the form of the API documentation and every method, class and construct is explained there are also many books on the subject. If you are a beginner I would recommend you start with Java.
C# might arguably be the second easiest to learn but doing so ties you to Windows Only.
C++ is arguably the hardest to learn of the languages named above and in the opinion of some one of the hardest languages to learn, period. C++ has compilers written for it for many platforms, if you learn to code in C++ on one platform it would be relatively easy to code on another - note C++ applications are not cross platform ready at compile time and many features such as GUI interfaces are non-standard.
C is in my opinion one of the simplest languages to learn as the language itself is tiny and incredibly compact. I have listed it as the least preferred to learn though as although the language itself is tiny the libraries and resources available for it are extensive and many more features such as GUI building or Memory Management etc are not part of the language itself and learning how to use all of those features can be a nightmare as C is aimed mainly at experienced developers who will know and understand their goals and be able to decide which library package would be best for their needs.