Selectbox in admin panel function linking to CSS

Not Recommended

  <?php if(theme_get_option('theme_style_options') == 'style1'): ?>
    <link rel="stylesheet" type="text/css" media="all" href="https://wordpress.stackexchange.com/questions/55754/<?php echo get_template_directory_uri(); ?>/style1.css" />
    <?php elseif(theme_get_option('theme_style_options') == 'style2'): ?>
    <link rel="stylesheet" type="text/css" media="all" href="<?php echo get_template_directory_uri(); ?>/style2.css" />
    <?php elseif(theme_get_option('theme_style_options') == 'style3'): ?>
    <link rel="stylesheet" type="text/css" media="all" href="<?php echo get_template_directory_uri(); ?>/style3.css" />
    <?php endif;?>

OR

<link rel="stylesheet" type="text/css" media="all" href="https://wordpress.stackexchange.com/questions/55754/<?php echo get_template_directory_uri(); ?>/<?php echo theme_get_option("theme_style_options');?>.css" />

Use wp enqueue style instead

function my_theme_styles(){ 
    $style = theme_get_option('theme_style_options');
    wp_register_style( 'custom-style', get_template_directory_uri() . '/css/'.$style);

    // enqueing:
    wp_enqueue_style( 'custom-style' );
}
add_action('wp_print_styles', 'my_theme_styles');