Answer: The right answer is the B. The Sevastopol Sketches.
Explanation: Just to elaborate a little on the answer, it can be added that the trilogy Childhood, Boyhood, and Youth was autobiographical, and the author started to write it when he was a young man serving in the Russian army, but it did not focus on his participation in the Crimean War. The Sevastopol Sketches (note the spelling), which he published in 1855, is the one based on his time serving in the Crimean War and set during the siege of the city of Sevastopol (1854-1855). Since the siege lasted almost a year, Tolstoy arranged his three stories by month, setting them in December, May and August.