Ответ: their, they're
Краткое пояснение: Выбираем притяжательное местоимение their (их) и местоимение they're (они есть).
- Look at the photo of my parents on their / there / they're wedding day. (their - притяжательное местоимение, указывает на принадлежность).
- You can see that there / their / they're really happy. (they're - сокращение от they are, указывает на состояние).
Ответ: their, they're