Chicken when referring to the bird is countable. So the plural would be chickens. However, when referring to the meat, chicken is uncountable. The correct answer should read "I saw ten chickens" and not "I saw ten chicken". Reported 2018-01-08
Now fixed (4 July 2018). It says "chickens" in the response options and "chickens" in the translation at the top of this page. Reporting pays off eventually!