The correct answer is $\ce{PhCOOH}$.
Decarboxylation is a chemical reaction that removes a carboxyl group ($\ce{-COOH}$) from a molecule. It is a common reaction in organic chemistry and is used in the synthesis of many different compounds.
The rate of decarboxylation depends on the stability of the carbocation that is formed. The more stable the carbocation, the faster the decarboxylation will occur.
In the case of $\ce{PhCOOH}$, the carbocation that is formed is a phenyl carbocation. Phenyl carbocations are very stable because the phenyl group is an electron-donating group. This electron donation stabilizes the positive charge on the carbocation, making it less likely to react further.
The other acids in the question do not have phenyl groups. This means that the carbocations that are formed are less stable and are more likely to react further. This makes decarboxylation of these acids slower than decarboxylation of $\ce{PhCOOH}$.
In conclusion, the acid that undergoes fastest decarboxylation is $\ce{PhCOOH}$. This is because the carbocation that is formed is very stable due to the electron-donating effect of the phenyl group.