Which one of the following is an assignment statement?

An assignment statement is a part of programming that assigns a value to a variable. In the context of the given options, the choice that qualifies as an assignment statement is the one that directly assigns a value to the variable without any additional context or keywords.

The statement "a = 20;" directly assigns the integer value 20 to the variable a. This is the basic syntax for assignment in many programming languages, where you have a variable name followed by an equal sign and then the value you wish to assign.

The other choices do not represent valid assignment statements in the same way. For instance, "int a = 20;" is a declaration statement combined with an assignment, where the variable a is first being declared as an integer before being assigned the value. The options that start with "assign" are not standard syntax in typical programming languages like Python, Java, or C++, therefore they do not qualify as assignment statements.