Here it says according to the solubility rules, which of the following ionic compounds will be insoluble. So if we start out with the first one, it's composed of sodium which is a group 1A element. Remember, anything connected to group 1A is automatically soluble, so we don't even need to look at the fact that nitrate is involved. If we did, Nitro would be the same thing. If it's present automatically, the compound is soluble. You don't need both to be present. As long as one of them is present, the whole ionic compound is soluble.
Next, we have calcium acetate. Acetate is one of the other ions that we discussed earlier. If it's part of the ionic compound, the ionic compound overall is soluble by default. Next we have barium sulfate. Remember, sulfate has exceptions. CBS Happy and here sulfates connected to barium. Barium is the B. In CBS we said that if sulfate is connected to CBS or HAP, it will be insoluble and form a precipitate. So C here is our answer.
Now if we look at the other choices, ammonium ion is part of this ionic compound, so by default it's soluble overall. And then here perchlorate is part of this ionic compound, so it too is soluble overall as default. So here the only option that would make an insoluble compound known as a precipitate would have to be option C.