(i) $H_2CFCOOH$ 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) $CH_2FCO_2H$ 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) $CH_3CHFCH_2COOH$ is stronger. Although both the given acids have F atom in them, it is the proximity of F in $CH_3CHFCH_2COOH$ to the COOH group which makes it more acidic.(iv) is more acidic due to the presence of electron withdrawing CF3 group in it.