💡 Detailed Explanation
The correct choice is 'ら' (forming 'あったら', the past conditional form '〜たら' of 'ある'). The adverb 'もし' (if) strongly pairs with the '〜たら' conditional pattern. Sentence translation: 'If there is time, would you like to go see a movie together?' The other choices: 'と' (creates 'あったと', meaning 'even if' or 'heard that'), 'なら' (requires dictionary/noun form), and 'ば' (requires provisional form 'あれば', not 'あったば') do not fit grammatically.