Answer:
How she had reached her conclusion.
Step-by-step explanation:
The word 'argument' implies the act of exchanging some form of differing opinions or diverging ideas about something or anything. It usually turns into a heated conversation, a loud and sometimes even violent discussion.
In the given sentence, the use of the word argument can best be understood when the speaker talks of "how she had reached her conclusion". Without this, it is quite unclear to understand what the argument meant, for "it was very difficult to follow" what the main issue was. But, with the ending phrase "how she had reached her conclusion", the readers can know and understand the use of the word "argument" in the sentence.