Final answer:
The correct way to complete the sentence is with 'starts,' as it refers to a scheduled event, which typically uses the simple present tense in English.
Step-by-step explanation:
The correct sentence to complete the statement "Let's go home. That wildlife documentary at 9:00 and I want to watch it." is:
4) starts
This is because the speaker is referring to a scheduled event, which is often expressed using the simple present tense in English. It indicates a fixed plan or a timetable, similar to saying 'the train leaves at 6 PM' or 'the store opens at 9 AM.'