Alternate to using if -else statements

What is the best alternative for a if --else if --block that contains many if statements.

code:

 string word="";

 if(letter=='a')
    {
       // do some action
       call(a);
    } 
 else if (letter=='b')
   {

           call(b)
   }

 else if (letter=='c')
   {
       call(c)
   }
 else if (letter=='d')
   {
       // do some action
       call(d)
   }
 else if (letter=='e')
   {
       // do some action
       call(e)
   }

    ................

     ..............

 else if (letter=='z')
   {
       // do some action
        call(z)
   }

If there are many if-else statements what is the best way to find a solution for such a scenario. Is there a way to effectively design such a scenario using classes/objects?

I am sure many of us would have faced such problems like this during design/coding. Is there a effective way to handle such a situation?

NO SWITCH STATEMENTS PLEASE. AN EFFECTIVE DESIGN ALGORITHM USING C# OBJECTS WOULD BE THE BEST BET.