When more water is added this raises the pH as the conc of H+ decreases. HF is a weak acid so its Ka at a certain temp must stay constant. When all of the concentrations of the things in the Ka equation decrease, something must increase to keep Ka the same, in this case, more of the acid dissociates, increasing conc. of H+ and A-, so percentage dissociated must increase.
However, as there's no salt of the acid in solution, it can't act as a buffer, at least not very well, especially when volume is doubled, so conc. of H+ overall decreases, and you have a lower pH. Hence, C.
Ka= ([H+][A-])
[HA]
For the second question, your values of [A-] and [HA] are the same, so your equation ends up as Ka = [H+], and Ka is the same as they're the same acid, so pH is the sam for both. However, as there is more [HA] and [A-] in M, it requires a greater volume of acid to change its pH than N, so N will change first. Hence, D