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.

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current