In which sentence is the capitalized word used correctly?

a) the principle welcomed the students back from vacation.
b) we visited the capitol building during our visit to washington.
c) the students were instructed to use capitol letters on the form.
d) my friend said that we should not participate, as it is a matter of principal.

i'm thinking the answer is d, but i am not 100% about it.

explain your answer clearly and thoroughly so i can understand. : )

you! ^-^

Why does the author's description of the mercury boiler—"the mercury, when vaporized, going into a mercury turbine and then into a condenser, remaining hot enough to generate steam in a steam boiler"—intentionally incorporate such highly technical language? using technical language establishes the expertise of the writer and convey the complex nature of the process being described. using technical language allows the writer to subtly poke fun at those who seek to make basic processes seem overly complicated. using technical language encourages readers to develop deeper and more meaningful personal connections to the information. using technical language lets the writer make clear to readers that the article is intended for readers who are themselves experts on electricity.