Most acidic compound of the following is:

The correct answer is option D, which can be attributed to the fact that due to large size of two -OCH₃ groups at ortho positions, there is steric inhibition of resonance (SIR), hence +I and +M induced electron donating effect of -OCH₃ group is not observed and thus acidity of the carboxyl , benzene ring increases.

While in option A, the methoxy group decreases acidity since it increases the electron density in conjugate base,through it +R effect.Similar,effect is seen by the OH group present at ortho position(s) in options B and C.

Hence, the most acidic structure amongst the following is given in option D, therefore,the correct answer is option D.