"It's six o'clock."
I should be thorough: numbers should be their respective kanji as well. So 6 should always be 六. ろく = slow
Although pronunciations are aimilar between kanji, hiragana, and katakana, they are used to discern between different meanings. Obviously hiragana is Japanese, kanji is used to separate distinct words (since Japanese doesn't have spaces), and katakana is used for loan words from other languages (usually). When talking about time, you always want to use 時
Normally Dulongo is very flexible with accepting various kanji/kana combinations for answers. I find that a good thing cause it help people in learning the kanji as well as the proper pronunciation. i.e. if it asks for 6 o'clock I would love to see duolingo accept "ろくじ", "六時" "ろく時" and "六じ". Even if it is normally mandatory to write it in kanji. One always ends up with the frustration of doing something wrong when in fact one didn't. i guess japanese #1 graders also start with hiragana only.