Happy to help!
I can see that you were caught by a common wrong answer. In RC questions about the primary purpose, there will often be answer choice that is too narrow to encompass the whole passage. Usually the test makers will make this answer even more tempting by including details or information from the end of the passage. Your answer choice was exactly that!
For the correct answer, we merely need to step back and look at the beginning again. The passage begins with pheromones, not vomeronasal. This is a clear indication that the passage is looking at a discussion of what is a pheromone and this vomer-thing is merely an example brought up to show the debate that is taking place about pheromones.
Does this make sense?
Always, always, always—make sure you know how the passage is organized before answering these questions. This passage goes from general to specific. And you will never find the primary purpose of a passage in the details. It will be in the general part.
I hope this helps you avoid these trap answers in the future.
Cheers,
Kevin
