This post relates to a past years paper… Paper 51 summer 2014
(i) Why is a salt bridge necessary?
My answer: To allow charge to flow, maintaining electrical neutrality in the half-cells.
(ii) Which (if any) of the following salts would be suitable to use in the salt bridge:
potassium chloride, potassium nitrate, potassium sulfate?
If you consider any to be unsuitable, explain why. 
Note 1: If the ions in the salt can form a ppt with any of the other ions in either of the half-cells, then it is unsuiotable.
Note 2: The question is asking about “THE cell” i.e. this specific one and not any random cell in general, so the ions in these particular cells must be considered.
Here we have a Ag(s)|Ag+(aq) half cell. Straight away, we must exclude any potential salt (for salt bridge purposes) with Cl- ions in them. Hence KCl is not suitable. It will REACT with Ag+ to form AgCl(s).
Depending on your graph, you probably deduced the M2+ was Pb2+ or Sn2+. So K2SO4 is not suitable as PbSO4 is very poorly soluble, so (SO4)2- ions would REACT the Pb2+ ions to form a precipitate.
BUT if you said the metal was Sn2+ then the K2SO4 would be an OK answer as SnSO4 is quite soluble 33g per 100g H2O, which is regarded as being highly soluble.
You could try and “always play it safe” and always choose KNO3, which is always soluble and eill never make a ppt, BUT make sure the nitrate ions cannot be reduced by some reducing agent! – otherwise it woo would react and probably give off a brown gas of NO2)
This question assumes you know the solubility of various species But I don’t think it’s in the syllabus! In case you’d feel better knowing the rules, they are…