"What year was last year?"
I believe they are the same, too. (Not a native though.)
Just that きょねん is more common in everyday conversation, but it's not good to write in a New Year's card because of an ominous kanji 去 which means "to pass away." So when you write "Thank you for last year" in your card, you use 昨年 instead.
Why does the hints always suggest words in kanji that does not exist among the examples to choose from? I don't understand kanji and I cannot learn them this way. There is no pronanciation of them. I'm just guessing and it's very frustrating. I need to hear the kanjis to understand them.