Translation:Tomorrow there is a debate between the five candidates.
I was wondering this too! A comment on another post says: "The ה turns into a ת when acting as a feminine marker at the end of a word in a construct state. A similar phenomenon occurs in Arabic." from https://forum.duolingo.com/comment/16317730?comment_id=28462746
Keep in mind that because מועמדים is plural masculine, the number, whether חמישה or חמשת is also masculine. It would be חמש if it were feminine.
The חמשת is just the smichut or חמישה, sort of like "the five of the candidates". It's a legitimate way to express it, although "חמישה המועמדים" is also acceptable.