The principle sets out three separate doors into a civil suit, the defendant's residence, the defendant's place of business, or the place where the cause of action arose, and a plaintiff only needs one of them open to file there. The job here is to see how many of those doors are actually open on these facts.
B, the defendant, carries on business in Mumbai, so that door is open. Separately, B's agent bought the goods and took delivery of them in Gurgaon, and that transaction is what generated A's claim for payment, so the cause of action arose in Gurgaon too, opening a second door.
With two independent grounds satisfied in two different cities, restricting the suit to only one of them, or throwing it open to the whole country, both miss the mark. The principle's own structure, any one of several qualifying places, is what makes Mumbai and Gurgaon both valid options together.
So the correct answer is At either of the places i.e. Mumbai or Gurgaon.
Asking what would need to change for each option to be correct exposes the gap in three of the four choices.
Since both qualifying grounds already exist without needing any change to the facts, and only one option reflects both together, that option is the one that fits.
Therefore, the correct answer is At either of the places i.e. Mumbai or Gurgaon.