The correct answer is A) The colonialist activities are largely ineffective.
One of the faults of imperialism that becomes clear in the "Heart of Darkness" is that the colonist activities are largely ineffective.
The "Heart of Darkness" is a novel written by Joseph Conrad in 1889. It refers to a trip in the Congo River to the independent state of Congo in what is known as the heart of Africa. The main character is a sailor called Charles Marlow that is the narrator of the story and has to deal with Kurtz, a well known and obscure ivory trader in the region.
The other options of the question were B) the activities of the colonialists aren't profitable, C) the colonialists get very lonely for their homes, and D) the colonialists are too focused on missionary work.