Final answer:
Nevada has the words Battle Born on its state flag, commemorating its entry into the Union during the Civil War.
Step-by-step explanation:
The state that has the words Battle Born on its state flag, having joined the Union in the Civil War, is Nevada (Option A).
The Civil War was fought from 1861 to 1865 between the northern states (Union) and the southern states (Confederacy) over issues including slavery and states' rights. Nevada joined the Union during the Civil War, officially becoming a state on October 31, 1864. The words 'Battle Born' on Nevada's state flag represent the state's entry into the Union during the war.