In 1813,
the Sixth Coalition attacked Napoleon and the First French Empire in the Battle of Leipzig (main clause),
the largest conflict in the Napoleonic Wars(modifies the Battle of Leipzig).
A. In 1813, the Sixth Coalition attacked Napoleon and the First French Empire in the Battle of Leipzig, the largest conflict in the Napoleonic Wars. - clear and concise construction - hold on
B. The Sixth Coalition attacked Napoleon and the First French Empire in 1813
, which was called the Battle of Leipzig and which became the largest conflict in the Napoleonic Wars - usage of which modifier is incorrect. also the parallelism in which modifiers implies that the Battle of Leipzig and the largest conflict in the Napoleonic wars are two separate entities. - Incorrect
C.
In the largest conflict in the Napoleonic Wars, in 1813, the Sixth Coalition attacked Napoleon and the First French Empire in the Battle of Leipzig - the opening modifier can either be a noun modifier (main subject) or clause modifier (action in main clause) - here the opening modifier is not fit for both - it shall actually modify the Battle of Leipzig
D. 1813 saw the Battle of Leipzig, the largest conflict in the Napoleonic Wars, in which the Sixth Coalition attacked Napoleon and the First French Empire
Awkward construction - 1813 saw ? is it possible

E. The Battle of Leipzig, the largest conflict in the Napoleonic Wars, was a conflict between the Sixth Coalition and the First French Empire in 1813 - omitted attacking Napoleon - also awkward construction - Incorrect
Answer A