Translation:Playing volleyball is my favorite sport.
The grammar is wrong in this English answer. It may follow what the Chinese statement says but it is not correct in English. I'm pointing this out because in other lessons they mark us wrong for not adhering to English grammar in the answers.
In order to fit English grammar, the answer should be either:
Volleyball is my favourite sport. Or Playing volleyball is my favourite pass-time .... or physical activity, or exercise.
As far as I know favorite when used as an adjective, comparative forms are not used because the word already has the 'most' meaning in it. It is probably not acceptable to say More favorite or Most favorite. So Duo's is a justifiable translation.
We can suggest an alternative like Playing volleyball is the sport I like the most.
2 things bother me:
Volleyball is a sport but is Playing volleyball a sport?
In Japanese they don't accept Sport for 体操, only Exercise; How come they translate 運動 to exercise now?