'Asa' is different from 'Kesa'. 'Asa' means any other morning than the current day's morning, which is 'Kesa'. So 'this morning' translates to 'kesa'. Any other morning translates to 'Asa'.
Kesa and Konban both start with K and mean THIS day.
Asa and Yuugata are any OTHER day.... YUUUGOTTHAT?
I translated that asa is "a morning" and duolingo wants it to be "the morning". Isn't asa just morning time in general, and not specifically one known morning?
Why do they first show us the kanji for "asa" with a teeny-tiny, impossible to actually see kanji (in the question where you select the image of "morning" from among three large images), but then throughout the rest of the questions, give it to you only in hiragana? Duo: please provide the kanji in a large-enough-to-see-all-the-little-lines, etc. format, and then please continue to incorporate the kanji throughout the lesson, rather than the hiragana.