Mathematical quiz game
In the Operation method

private static string Operation(int randomIndex)
{
    string operation = "";
    switch (randomIndex)
    {
        case 1:
            operation = "+";
            break;
        case 2:
            operation = "-";
            break;
        case 3:
            operation = "*";
            break;
        case 4:
            operation = "/";
            break;
        default:
            Console.WriteLine("error");
            break;
    }
    return operation;
}


instead of doing operation = ... and then returning operation you can just return 

Example :

case 1:
    return "+";


Same thing in the Cal method.

In the CheckAnswer method you should just have else instead of else if(userInput != sum)

Also in the Main your Random rnd = new Random(); should be out of the do while loop.