I think your best choice would be a, but it can very well be d.
Answer:
jxxxnujsWlfo2znbj9jinunybrcnx
Most often, it is deciphered through A) context clues. Some people, however, choose to decipher them through B) pronoun references. Just to be on the safe side, I'd choose A) context clues because most often, the words that are being deciphered are they themselves the pronoun.
Answer:
C. Situational
Explanation:
Since you've studied, thinking it wasn't going to be open book/note, you didn't need to study as hard and feel like it went to waste. So in this case, the irony is situational.