Of these nonmetals, which one is likely to be the least reactive? nitrogen (n) phosphorous (p) sulfur (s) oxygen (o)

Out of these options, I would go with nitrogen.

First of all, I would eliminate sulfur and oxygen right of the bat because they have 6 valence electrons, while nitrogen and phosphorus have 5. With 4 valence electrons being a neutral element, 5 would be the next most neutral, meaning, the next most least reactive. Then, I would eliminate phosphorus because it has 3 energy levels, while nitrogen has 2. This is a contributing factor since the smaller the element, or closer the electrons are to the nucleus, the more stable the element becomes.

Hope this helps!