OP is referring to the audio. It's unfair that the correct answer is 'joli' when both masculine and feminine variants sounds exactly the same.
C'= ça or cela = indeterminate gender. Both "joli" and "jolie" should be accepted.
c' or ce or ceci or cela are not indeterminate gender, they are masculine (by default), so the following adjective has to be in masculine.
My mistake. C'est should always be followed by the masculine singular adjective, correct?