Which of the following acid has the highest acid strength? (A)...

Acid strength is directly proportional to the stability of the conjugate base, which increases with the presence of electron-withdrawing groups (-I, -R effects).

In fluoroacetic acid ($F-CH_2-COOH$), the highly electronegative fluorine atom exerts a strong -I effect from the adjacent $\alpha$-carbon, highly stabilizing the conjugate base. Its $pK_a$ is approximately $2.59$.

In 4-nitrobenzoic acid, the $-NO_2$ group exerts both -I and -R effects, but it is located farther away on the benzene ring. Its $pK_a$ is approximately $3.41$.

Acetic acid ($pK_a \approx 4.76$) and 4-methylbenzoic acid ($pK_a \approx 4.34$) have electron-donating groups (+I and hyperconjugation), which decrease acid strength.

Comparing the $pK_a$ values, fluoroacetic acid has the lowest $pK_a$ and is therefore the strongest acid among the given options.

Answer: (B)