"What is popular?"
Translation:Τι είναι δημοφιλές;
So, you uses "Τι" when you dont know the number of what you asking about and "Ποιο" when you have something to counting on?
The basic form is not δημοφιλός but rather δημοφιλής — different declension.
Why then does the answer assume a plural, and mark the singular form a typo? https://screenshots.firefox.com/nNEAfcrkNuAffZcE/www.duolingo.com
Both δημοφιλής and δημοφιλές are singular -- the first is masculine or feminine, the second is neuter.
The plural would be δημοφιλείς (masc/fem) or δημοφιλή (neuter).
See e.g. https://en.wiktionary.org/wiki/%CE%B4%CE%B7%CE%BC%CE%BF%CF%86%CE%B9%CE%BB%CE%AE%CF%82 (and open up the declension table there).
τι is neuter, so you need the neuter form of the adjective here, not the masculine/feminine one.
Much as you would ask τι είναι καλό; for "what is good?" and not τι είναι καλός; or τι είναι καλή;.