A void return type means that you tell the compiler "this function returns nothing", yet you have a return(letter) at the end of the function.
You are using old-style declaration of arguments - but i think you meant to declare l1, l2 and l3 as local variables - which leads to the question of how you wish to get the answers out of the function. Perhaps you want to pass a set of pointers to float to the function, and fill those in...
float l1, l2, l3;
Line 79 error 6
printf("Enter the length of side A");
scanf("%f%*c", &l1); line 82 error 7
printf("/nEnter the length of side B");
printf("Enter the length of side C");
"return (char letter)" is definitely very wrong. If we assume that it was compilable, then it would mean "create a variable of type char called letter, then return it" - without giving it any value, so it would return rubbish.
The function getquestions() hasn't been declared before DoOption, so it complains about that too.