In the context of elections and national conventions, what is a delegate?

a$)$ the second$-$place finisher in a primary contest; should the first$-$place winner be unable to serve, the delegate serves as their replacement

b$)$ a representative who votes according to the preferences of his or her constituency

c$)$ the person selected by a political party to run in an election to win office

d$)$ party leaders who vote according to their own preferences in a presidential nomination