OK, got it. Not necessarily the best wording, I was all the time thinking about dilution that takes place when solutions are mixed.
Not only wording is poor, also answer is incorrect. Usually I would help you to solve the question on your own, but I have no idea what the expected answer is.
The only solution that has constant pH is the last one - that is, ammonium acetate.
Solution A: pH after mixed 4.67, after diluted with 100 mL of water - 4.69
Solution B: pH after mixed 8.63, after diluted with 100 mL of water - 8.56
Solution C: pH after mixed 4.67, after diluted with 100 mL of water - 4.69
Solution D: pH after mixed 7.00, after diluted with 100 mL of water - 7.00
You may have troubles getting these numbers, as they take ionic strengths into account (I have calculated them using this
pH calculator). When ionic strengths are ignored calculated pH values are:
Solution A: pH after mixed 4.76, after diluted with 100 mL of water - 4.76
Solution B: pH after mixed 8.71, after diluted with 100 mL of water - 8.63
Solution C: pH after mixed 4.76, after diluted with 100 mL of water - 4.76
Solution D: pH after mixed 7.00, after diluted with 100 mL of water - 7.00
A & C are buffers, so their pH is more or less constant. Solution B is a solution of salt of a weak acid, which is equivalent to solution of a weak base, its pH is obviously dependent on the concentration. Solution D is a funny case of solution of a salt of a weak acid and a weak base - funny, as they are both weak in exactly the same degree, so effects of their hydrolysis cancel out.