The correct sentence is (A) The students have finished their homework early today. This sentence uses the present perfect tense (have + past participle), which is appropriate for an action completed within a time period that is still ongoing ('today') and has relevance to the present moment. The auxiliary verb 'have' correctly agrees with the plural subject 'The students'.
Option (B) 'The students finished their homework early today' uses the simple past tense. While grammatically plausible, the present perfect (A) often provides a more precise nuance when the action's completion is recent and within an uncompleted timeframe ('today'), emphasizing the present result. Option (C) 'The students has finished their homework early today' is incorrect due to subject-verb agreement; 'has' is used with singular subjects, whereas 'The students' is plural and requires 'have'.