Step 1: Context.
Louis XVI of France was executed during the French Revolution by guillotine.
Step 2: Exact date.
He was executed on 21 January 1793 after being found guilty of treason.
Step 3: Eliminate options.
(A) 1748: Too early, before the French Revolution.
(C) 1798: Wrong, revolution events had already moved ahead.
(D) 1854: Too late.
Step 4: Conclusion.
Correct answer: (B) 1793.