Explanation:

- Option 1: slain

- "After the militants slain they were buried" is grammatically incorrect. "Slain" is the past participle form and not suitable here without an auxiliary verb.

- Option 2: were slain

- "After the militants were slain they were buried" is grammatically correct. "Were slain" uses the passive voice correctly to show that the action happened to the militants.

- Option 3: got slain

- "After the militants got slain they were buried" is informal. Though understandable, it is less formal and standard than "were slain."

- Option 4: slay

- "After the militants slay they were buried" is present tense and does not make sense in this past context.