What can be concluded about the atom from knowing that oxygen-18 has an atomic number of 8?

I use this method to remember:
Let's use A as the mass number, and A = protons + neutrons 
Z is the atomic number, which is protons
To get neutrons, you subtract A from Z.
So in this sense, neutrons = 18 - 8 = 10 
I hope this helps with your work. :)

Ithink the answer is b.

The mole ratio of hydrogen to ammonia is 3: 2.