$question[15] = "Picanha, um corte da carne bovina, é um prato tipicamente...";
$answer[15][0] = array("Brasileiro", true);
$answer[15][1] = array("Português", false);
$answer[15][2] = array("Mexicano", false);

//A better array would be something like
$answer[15]["Brasileiro"] = true;
$answer[15]["Português"] = false;
$answer[15]["Mexicano"] = false;

//or even just
$answer[15]["Brasileiro"] = true;
//ignoring any false answers

//both of these are using associative arrays, where the key is the answer string.

//then you can just do something like:
$response = $_REQUEST[ 'html_input' ];
//$response would now look something like "Brasileiro"

if( $answer[15][$response] === true ){
    echo "Congratulations! you got the question right!";
}