Answer:
The conflict is between the man/the narrator and his own self thoughts.
Step-by-step explanation:
O Henry's "The Cactus" is a short story that revolves around the protagonist Trysdale's thoughts, about his marriage and how it went awry. This short story also delves into the protagonist's own character.
Even though there are other characters mentioned and involved, the main conflict is primarily within himself. He did not try to understand the cactus nor did he seem interested in trying to. So, the woman who was supposed to be his wife decided to not marry him, discovering that he is an egoistic man and nothing, and no one can or will be able to change him. This left him wondering how/ why she refused to marry him, never realizing his own mistake.
Thus, the correct answer is that the conflict occurs between the man and his own self.