Hi Arpitpatel86. I understand your question about spectator ions, and yes, they are at times difficult to notice, but its doable. =) Let's take a look and take it one step at a time.
First, what are spectator ions?
Spectator ions are atoms (charged) that do not participate in the reaction. That means they will appear in both the reactants and products, and will not participate in the chemical reaction. For example, think of it like us! When we volunteer at dental offices, we are not participating in the actual procedure to be performed on the patient - we are spectators. Even though we may help out, we are not directly participating in the procedure. We are spectators like the crowd at the Lakers game.
How do you know which are spectator ions?
In order to understand this aspect, and the answer to your question, you need to be aware of solubility rules. As the name implies, it pertains to the appearance of a precipitate at the completion of the reaction. You can find solubility rules in the KAPLAN book, or any chem book would suffice.
Now lets take your choices one at a time:
KBr
"Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+).Salts containing Cl -, Br -, I - are generally soluble. " - This is the definition straight from my book.
NaF
If we look in the second line of the quote above, we see that F is not one of the choices. Therefore, F would participate in the reaction, but how?
F + H2O --> HF + OH-
Please follow this technique for choices C to E, and lets get to the answer. Great!
"Salts containing nitrate ion (NO3-) are generally soluble."
Which means we only have NH4+ left. This species will donate its proton to water forming H3O+, and hence the solution would be acidic (power of Hydrogen (pH) less than 7).
I'll check later on to see if you are still unclear.
