Which of the following is a strong reducing agent?

Statement-$I$: Most carboxylic acids exist as dimers in vapour phase or in aprotic solvents due to intermolecular hydrogen bonding.
Statement-$II$: Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of the hydrocarbon part.

Oxalic acid reacts with concentrated $H_2SO_4$ to give a mixture of two gases. When this mixture is passed through caustic potash,one of the gases is absorbed. What is the product formed by the absorbed gas with caustic potash?

When succinic acid is heated,the product formed is:

Which of the following can undergo Hell-Volhard-Zelinsky reaction?

Which of the following is most stable?