D: a warship fires at the land, even though there is nothing there to shoot.
Explanation:
"Heart of Darkness" (1899) is Joseph Conrad's masterpiece novel. The major themes of the novel are imperialism, colonialism, and racism.
There are many examples of colonials ineffectiveness throughout the novel. The selected option is just one instance. Another famous one is in the leveling of the cliff for no apparent purpose. The narrator satirizes that may be it was just for philanthropic purpose of keeping the slaves busy. A pointless attempt to extinguish a burning grass hut is one more such example to show colonials inefficiency in Africa.
Dictionaries are intended to be convenient, therefore they would put the most common definition first to avoid the reader to search through all the other ones.