The answer is c) treat. Joy is used to describe an emotional state. I can say: Dance brings me joy. Dance puts me in that particular state. I can also say: She is a joy to have around. This means she causes or brings the feeling of joy, or she is a source of joy. Bliss is also an emotional state, and now we use this word to say things like: Find your bliss. This means find the thing that brings you happiness. Treat here is like a surprise or a fun event. I could say: For a special treat, I am making my husband's favorite meal. The meal may bring my husband joy, but I can't say: For a special joy, I am making my husband's favorite meal. Treatment does not really fit with the other words in this case. Treatment can be used to talk about the kind of service you receive. For example: I received terrible treatment at that hospital, and I never want to go back. We also describe some types of medical and psychiatric and psychological care as treatment. I hope that helps! Also, great question!