Constitution ability check or Constitution Saving Throw?

Lunging Attack + Sentinel, monster moves around PC in a 10ft, do attack of opportunity happens?

Slow spell, do you need to have line of sight on creatures to affect them?

Can a Monk use either Dex or STR for grappling attacks?

Lucky Feat, The second use on the enemy attacks roll, which one of these statement are true?

Perception vs Investigation, more detailed answer?