The correct choice is 'たちます' (from '経つ' - to pass/elapse [of time]). Sentence context: 'It has already been half a year since I came to Japan.' The verb '経つ' (tatsu) specifically refers to the elapsing of time. The other options: 'おわります' (to finish), 'します' (to do), and 'あります' (to exist/have) are not used to collocate with a period of time to mean 'time has passed'.