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

Phosphorous

Explanation: Nitrogen(N): 7:

Phosphorus(P): 15:

Oxygen(O): 8:

Sulphur(S): 16:

As nitrogen contains half filled configuration, it is stable and does not react. Phosphorous lies below nitrogen and reactivity decreases down a group for non metals as they tend to gain electrons as the valence shell move away from the nucleus.

Oxygen has 4 valence electrons, thus it need to gain two electrons to attain stable configuration. Thus is reactive.

Sulphur lies below oxygen and also need to gain two electrons. Reactivity decreases down a group for non metals as they tend to gain electrons as the valence shell move away from the nucleus.