To solve this question, the user needs to know the concept of subsets, i.e., a subset is a part of a larger set. The user must identify which language(s) are a subset of C++.

Now, let's go through each option and explain why it is right or wrong:

A. C language: This option is correct. C++ is an extension of the C language, which means that all C programs are also C++ programs. Therefore, C is a subset of C++.

B. Java Language: This option is incorrect. Java is not a subset of C++. Although both languages share some similarities, they are distinct programming languages with different syntax and features.

C. C# language: This option is incorrect. C# is not a subset of C++. C# is a programming language developed by Microsoft and is not related to C++.

D. B language: This option is incorrect. B language is not a subset of C++. B language was developed in the early 1970s and is not widely used today.

The Answer is: A. C language