Why does more than one story arc make a novel interesting?

More than one story arc make a novel interesting:

Having more than one story arc in a novel or an any other story makes the entire plot more interesting and the readers can focus more than one plot and would not feel bored. By this way, the writer can also include more details and throughout the arc of the story the momentum of the pace shall be maintained. Which is the very reason why authors prefer to have more than one story arc in a novel. Simply to maintain momentum and to engage the readers to give better experience.

b is the answer for this question.