(i) H2CFCOOH will be stronger of the two. The presence of electronegative F atom at the α-C causes electron withdrawal from the COOH and facilitates the release of H‌+.
(ii) CH2FCO2H is a stronger acid for the same reason as stated above. F is more electronegative than Cl, so it withdraws electrons from the carboxyl group to a greater extent.
(iii) CH3CHFCH2COOH is stronger. Although both the given acids have F atom in them, it is the proximity of F in CH3CHFCH2COOH to the COOH group which makes it more acidic.
(iv) is more acidic due to the presence of electron withdrawing CF3 group in it.