In C#, the variable named "this" is used to refer to the current object whose method is being invoked.

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

A. call: This option is incorrect because "call" is not the variable used to refer to the current object in C#.

B. this: This option is correct. In C#, "this" is used as a reference to the current object. It allows you to access members of the current instance within a method or constructor.

C. do: This option is incorrect because "do" is not the variable used to refer to the current object in C#.

D. that: This option is incorrect because "that" is not the variable used to refer to the current object in C#.

Therefore, the correct answer is B. this.

Let me know if you have any other questions or if there's anything else I can assist you with!