Which of the following is a valid way to declare a function in C?

The answer is valid because it follows the correct syntax for declaring a function in C. In C, a function declaration consists of the return type, followed by the function name, and then a parameter list enclosed in parentheses. The return type for this function is specified as "int," indicating that the function will return an integer value. The parameters "int a" and "int b" are listed in the parentheses, signifying that the function takes two integer inputs.

The other choices do not conform to the C language syntax for function declarations. For instance, the second choice incorrectly places a colon after the parameter list and before the return type, which is not how C function declarations are structured. The third option uses the "->" syntax, which is not valid for function declarations in C; this syntax might be observed in other programming contexts, such as with pointers or in other languages. Lastly, the fourth choice introduces the term "declare," which is not used in function syntax within C. The text appears to mix function declaration with incorrect syntax elements, leading to an invalid declaration.