I am using this plugin for Hidden Answer Quiz.
After searching around, I discovered this is the only Wordpress Hidden Answer plugin in the market, perfect for Hidden Answer Quiz, Reveal Hidden Coupon code, Hide and Reveal FAQs and so on. Initially, I was looking for something like a scratch off sticker that is used on coupon sites like savings.com, but this plugin does the job of hiding answers as well, so it's a good option.
I hope the developer will add more features such as a multiple choice quiz which is linked to the hidden answer. In this case, the hidden answer should not be revealed unless the user completes the multiple choice question by selecting/guessing/putting his or her own answer.