Final answer:
The statement c = a + b can be achieved by directly using c = a + b; for string concatenation in programming. Other provided options are incorrect for concatenating strings.
Step-by-step explanation:
The student's question relates to string concatenation in programming, specifically when combining two String variables a and b into a third String variable c. The correct way to achieve c = a + b; is using the expression c = a + b;, which directly concatenates strings a and b. The options provided, such as c = () = ();, c = (int)a = (int)b;, c = (b); or c = (a); are either syntactically incorrect or do not achieve concatenation in any programming language.
Commutativity (A + B = B + A) applies to ordinary number addition, but when dealing with strings, the + symbol represents concatenation, and the order of the operands matters.