Answer:
A. If it were not for them being alone in the cab with time to spare, the stories main plot would not be resolved.
Step-by-step explanation:
O. Henry's short story "Mammon and the Archer," tells the story of a rich man and his life of money and how he helped save his son's life. The story follows a young man trying to get his love and how his father had secretly helped save him from all that stress and 'sickness' of being love-struck.
The setting in the cab is the main point of the whole story, the deciding factor for the plot's resolution. So, if it were not for the two lovers being alone in the cab, then there would have been no time to spare which will lead Miss Lantry to be with her mother and not Richard. Also, it was the delayed act with the cab scene that helped the plot's resolution, or else the resolution may have turned into something different.
Thus, the correct answer is option A.