Which one of the following will undergo decarboxylation on heating?

Which of the following compounds has the maximum acidic strength?

Dehydration of an organic acid $X$ with concentrated $H_2SO_4$ at $373 \ K$ gives $H_2O$ and gas $Y$. The hybridisation of the carbon in $Y$ and nature of $Y$ are respectively

Among the following compounds,the most acidic is

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

What is formed when oxalic acid is dehydrated with concentrated $H_2SO_4$?