To determine which of the given compounds is the strongest acid, we need to consider the effects of the halogen substituents (F, Cl, Br, I) on the acidity of the carboxylic acid group. The acidity of carboxylic acids is influenced by the stability of the negative charge formed on the conjugate base after deprotonation.
1. **FCH₂CO₂H (Fluoroacetic acid)**: The fluorine atom is highly electronegative and tends to withdraw electron density through the inductive effect. This decreases the stability of the conjugate base (FCH₂CO₂⁻), making it a stronger acid than acetic acid.
2. **ClCH₂CO₂H (Chloroacetic acid)**: Chlorine is less electronegative than fluorine, leading to a lesser degree of electron withdrawal. Although chloroacetic acid is still a stronger acid than acetic acid, it is weaker than fluoroacetic acid.
3. **BrCH₂CO₂H (Bromoacetic acid)**: Bromine’s electronegativity is less than chlorine’s, resulting in a further decrease in acid strength compared to chloroacetic acid. Thus, it is weaker than both fluoroacetic and chloroacetic acids.
4. **ICH₂CO₂H (Iodoacetic acid)**: Iodine is even less electronegative than bromine, which means it has the weakest inductive effect among the four halogens considered here. Therefore, iodoacetic acid is the weakest acid in this series.
Based on these considerations, fluoroacetic acid (FCH₂CO₂H) is the strongest acid among the given options due to the strong electron-withdrawing effect of the fluorine atom, which stabilizes the conjugate base more effectively than the other halogens.