HTML dropdown setting not showing last saved value in the database

The problem is this:

selected(get_option('lp_actPage', $slug))

selected needs 2 parameters, but you gave it 1, and get_option wants 1 parameter but you gave it 2.

If we reformat this, the problem becomes much clearer:

selected(
    get_option('lp_actPage', $slug)
)

What’s necessary is:

selected(
    get_option('lp_actPage')
    $slug
)

Sidenote

get_pages doesn’t use the normal post querying code, and doesn’t benefit from the same enhancements and plugin integration you get from other functions.

Instead of get_pages, consider:

$pages = get_posts( [
    'suppress_filters' => false, // enable caching
    'post_type' => 'page', // we want pages
    'post_status' => 'publish', // speed up by asking for only published posts
    'posts_per_page' => 100,
] );

Likewise, don’t use <? use <?php or your code will break when your host upgrades PHP in the future.