Which of the following novels is NOT written by Amitav Ghosh?

The correct option is English, August.

To understand why this is the correct answer, we can examine the authorship of each of the novels listed in the options:
1. Sea of Poppies (published in 2008) is the first novel in the acclaimed Ibis Trilogy written by Amitav Ghosh. It was shortlisted for the Man Booker Prize in 2008.
2. Flood of Fire (published in 2015) is the third and final novel in Amitav Ghosh's Ibis Trilogy.
3. Gun Island (published in 2019) is a contemporary novel written by Amitav Ghosh that explores themes of climate change, migration, and folklore.
4. English, August (full title: English, August: An Indian Story, published in 1988) is a celebrated novel written by Upamanyu Chatterjee, not Amitav Ghosh. It follows the life of a young Indian Administrative Service officer named Agastya Sen.

Since English, August is authored by Upamanyu Chatterjee, it is the novel in the list that was NOT written by Amitav Ghosh.