It's clear that the narrator is on a kind of spiritual quest, but the nature of that "quest" includes these other ways of describing the story. Zaabalawi may or may not be a real person, but it is the narrator's belief in his healing powers, bolstered by the other believers he encounters, that leads him on. The act of searching for him is more important—and more beneficial—than actually finding him because it is through his interactions with men like the calligrapher and the musician that he comes to into contact with a community of believers that confirm the worthiness of his search.

The narrator, eventually, comes to define himself through the search. Whether this form of self knowledge extends to obsession or mental illness is not clear. While one can read the narrator's dream as a kind of hallucination or spiritual vision of Zaabalawi, from the evidence provided by the story (in which Wanas explains that Zaabalawi had turned up while he slept) is inconclusive. Zaabalawi may or may not be a real person; everyone claims to know him, but he is never seen; the narrator's faith in him and desire to find him shapes his life. The healing he craves, perhaps, is achieved not by finding Zaabalawi, but in seeking him.

The possible answers provided in the question are not mutually exclusive. Evidence can be found in the story for all of them, and any individual reader’s evaluation of these themes will primary will depend on the supporting evidence they find. The key words in each answer are very similar: "quest," "search," and "pursuit." Similarly, "obsession" and "desperate" could be considered closely related. While some people might perceive spirituality as the central theme of the text that motivates the characters, others might consider self-knowledge more, and how it is broadly applicable to human interior states.

The narrator discovers he has an incurable illness, which prompts the quest on which he embarks; so, considering the major themes in this text could be related to the narrator's coping mechanisms for dealing with this situation (as opposed to giving in to despair). There is an obsessive quality to the quest, as he never stops searching for Zaabalawi. In an altered state, through an alcohol-induced vision, he "finds" his quarry. Part of the self-knowledge he seeks is, paradoxically, whether or not he can accept defeat in his search or eventually achieve satisfaction with his lot in life. The story can be seen as a contrast between idealism and realism in one’s attitude toward life.

While all of these answers have some validity, the most convincing answer is that the story "Zaabalawi" can be interpreted as answer A, a spiritual quest. The narrator is "afflicted by that illness for which no one possesses a remedy." He is "overcome with despair," so he goes in search of Sheikh Zaabalawi, a saint who takes away people's troubles. 

Most people the narrator encounters are disinterested in finding Zaabalawi. The lawyer, named Sheikh Qamar, does not even recall the saint, as the lawyer lives a life of material luxury that occupies his mind. The shopkeeper, who sells books of theology and mysticism from the entrance of a dilapidated house that is filled with garbage, is too tired and careworn to search for the saint. The sheikh of the district approaches the task of finding the saint in a systematic way that deprives it of its spiritual quality. He says he has been too preoccupied with "the cares of the world" to think about the saint lately. The only people who seem interested in finding Zaabalawi are the calligrapher and musician, who are both spiritual men.

When the narrator goes to a bar in search of the saint and gets drunk, he has a beautiful and serene dream, and when he wakes, he hears that Zaabalawi has been with him. In his state of drunkenness, he has entered a spiritual state in which the saint can visit him. He vows to continue to search for Zaabalawi at the end of the story. It is clear the saint is not a material presence but a spiritual presence who only visits people who are spiritual in nature and who are carried away, as the narrator was by drink, into a state of ecstasy or mysticism. The other characters are too focused on earthly matters to find the saint, but the narrator, carried into the spiritual realm, finds his saint. The tale is one of a spiritual quest.