The . . . . . . . . keyword is necessary only in a dual-alternative selection structure.

The correct answer is: D. Else

The else keyword is necessary only in a dual-alternative selection structure. A dual-alternative selection structure is a type of control flow statement that allows you to choose between two possible actions. The else keyword is used to specify the action that should be taken if the condition in the if statement is false.

The alternative keyword is not necessary in any type of selection structure. The next keyword is used to move to the next statement in a loop. The dual keyword is not a valid keyword in C#.

Here is an example of a dual-alternative selection structure:

c#
int x = 5;
if (x == 5) {
Console.WriteLine("x is 5");
} else {
Console.WriteLine("x is not 5");
}

In this example, the if statement checks if the value of x is equal to 5. If it is, the Console.WriteLine("x is 5") statement is executed. If it is not, the Console.WriteLine("x is not 5") statement is executed.