To construct a timeline of significant events that led up to the formation of the Roman Empire in chronological order, the correct sequence would be:
- Gracchi brothers' assassinations (133–121 BCE) - The social reforms and subsequent violence highlighted the political tensions in Rome.
- Sulla's march on Rome (88 BCE) - This marked a significant moment of conflict and the use of military force in Roman politics.
- First Triumvirate (60 BCE) - An informal political alliance between Julius Caesar, Pompey, and Crassus to consolidate power.
- March across the Rubicon (49 BCE) - Julius Caesar's crossing was a declaration of war against the Roman Senate.
- Ides of March (44 BCE) - The assassination of Julius Caesar, which led to further civil unrest.
- Second Triumvirate (43 BCE) - An official alliance between Octavian, Mark Antony, and Lepidus to defeat Caesar's assassins.
- Battle of Actium (31 BCE) - The decisive naval battle between Octavian and Antony, leading to Octavian's control of Rome.
Given this timeline, the correct answer is:
D. Gracchi brothers' assassinations, Sulla's march on Rome, First Triumvirate, march across the Rubicon, Ides of March, Second Triumvirate, Battle of Actium.