Simple temperature converter in C
Use an enum instead of #define. So instead of:

#define option_1 1
#define option_2 2
#define option_3 3
#define option_4 4


You can use enum:

enum {F_to_C, C_to_F, Off} option_type; // I am leaving off the fourth option!


You will also have to modify other portions of your code to use enums.

Stop using so many if statements. Use switch-case statements instead. There are a few places that you do this in your code but for example:

if (option == option_1)
    printf("Farenheight to Celsius of %.1f F is: %.1f C\n", input_temp, F_to_C(input_temp));

else if (option == option_2)
    printf("Celsius to Farenheight of %.1f F is: %.1f F\n", input_temp, C_to_F(input_temp));

else if (option == option_3)
    printf("Calculator OFF\n");

else if (option == option_4)
    printf("Invalid option choice, please try again...\nOption: ");


Instead:

switch(option) {
    case F_to_C:
        printf("Farenheight to Celsius of %.1f F is: %.1f C\n", input_temp, F_to_C(input_temp));
        break;
     ...
     default:
         printf("Invalid option choice, please try again...\nOption: ");
         break;


Notice that we have a default. You can give garbage as option_4, more than just the value 4. You should try handling this best you can. 

Don't use _Bool! Just use bool and use true and false instead of 1 and 0. So this:

_Bool calc_off = 0;


Should be:

bool calc_off = false;


But actually I don't really believe you need calc_off because you use pointers unnecessarily.

This:

int option = 0;
float input_temp = 0.0f;
_Bool calc_off = 0;

while (!calc_off)

{
    get_input(&amp;calc_off, &amp;option, &amp;input_temp);

    if (option == option_1) /*F to C*/ // Rewrite this to use switch case!
    {
        display_result(option_1, input_temp);
    }
    else if (option == option_2) /*C to F*/
    {
        display_result(option_2, input_temp);
    }
}


get_input in particular is troublesome.

Why not let get_input return an option? then:

while((option = get_input(...)) != Off) {