How to access a variable on all page?

Generally, you cannot access local variables in a function from outside the function, so this…

function func() {
    $var = 2;
}

$var = 1;
func();
echo $var;

would result in “1”.

To make a global variable, you can do this to make $option available outside functions.php:

function func() {
    global $option;
    $option = get_option( 'simple_options' );
} 

However, I cannot recommend it, since every other script may change the value. You can use a define:

define('MY_OPTION',get_option( 'simple_options' );

And in your footer:

echo MY_OPTION; // note that there are no quotation marks

But also this wouldn’t be that neat, since you’d have to make sure no other script defines this MY_OPTION and you cannot change it later on.

No, your best bet is to get the option just again in your footer.