Hey guys, I have another question sorry 
http://m.imgur.com/s0p8ByO
For Q17) why isn't A correct?
A states that H2SO4 is ACTING (key word) as a strong acid. Yes, it IS a strong acid, but in this reaction, it isn't ACTING as a strong acid. The key is to notice the Magnesium - if you EVER have a metal go from M to MX (i.e., some metal salt), then it WILL have changed oxidation states, and the reaction WILL be redox. Redox reactions are NOT acid base reactions (at least, as far as VCE is concerned. The curious should google Lewis acids/bases, but don't confuse them with Bronsted and VCE!!). Since we know from looking at Mg (which goes from 0 -> +2 in oxidation state), we know that the reaction is redox, and hence NOT acid-base. If it's not an acid-base reaction, then H2SO4 CAN'T be acting as an acid. Instead, it's our oxidant.
Q18) how is the answer B? I don't get where the HCl comes from?
We use the electrochemical series.

Remember, spontaneous reactions happen with higher left to lower right compounds.
So, check A - Pb MUST be more reactive than metal X. Not true - in fact, for Pb^2+ + X ----> Pb + X^2+, then Pb must be LESS reactive than X.
Now, B. The important part of HCl to H2 for electrochem is the H+. H+ is only slightly more reactive than Pb2+, so this very quickly seems like a good shot. This could be right, but for completions sake, let's check the others.
Next, C. Metal X must be able to react with Mg(NO3)2 to give Mg precipitate. Not true - fact, if X is Sn, Ni, Co (and so many others), then we will react with Pb^2+, but NOT Mg^2+.
Finally, D. Pb must be able to react with X(NO3)2. This is kinda true - if we supply an electric current, we will get electrolysis, by the nature of electrochemical reactions. However, this is less correct than B, because B is a spontaneous reaction, and it's implied that no current is used by the question.
So, A, C and D are all wrong, and the answer must be B. They try to throw you off by mentioning HCl, when it wasn't in the quesiton. BUT, by going to the electrochemical series, you can see why that might come into play.
EDIT: Beaten by sweetcheeks.

Leaving this here as I went through each option and why they're right/wrong.