Your list is missing Giratina, Poipole, Naganadel, Stakataka, Blacephalon, and Zeraora. Type:Null and Silvally are debatably legendary, at least the games consider them as such.
1. Giratina si missing, as well as other 7th generation pokémon as other people already noted;
2. It is not essentially correct to enlist "Ultra Necrozma", as well as "Dusk Mane" and "Dawn Wings", as legendary pokémon, since they only are alternate forms of other existing legendary (if you want to add alternate forms, then you should also add Mega Rayquaza, Archeo Kyogre, Archeo Groudon, Origin Giratina, Black and White Kyurem, and so on...);
3. It would be more helpful if you enlist the legendary pokémon following the pokédex order;
4. Answers are not well cared for, as some of them have capital letter, some not. I would expect much more precision and caring in creating a quiz.
2. It is not essentially correct to enlist "Ultra Necrozma", as well as "Dusk Mane" and "Dawn Wings", as legendary pokémon, since they only are alternate forms of other existing legendary (if you want to add alternate forms, then you should also add Mega Rayquaza, Archeo Kyogre, Archeo Groudon, Origin Giratina, Black and White Kyurem, and so on...);
3. It would be more helpful if you enlist the legendary pokémon following the pokédex order;
4. Answers are not well cared for, as some of them have capital letter, some not. I would expect much more precision and caring in creating a quiz.