You might consider changing the if else structure to a switch statement, or even consider calculating simple power or adding voltage and current division for series or parallel circuits.

Code:
...
scanf("%d", &choice)
switch(choice)
{
   case 1: voltage();
           break;
   case 2: current();
           break;
   case 3: resistance();
           break;
   default: // invalid entry
           break;
};
AC power is a little more involved