$$ \text { The following structures are } $$

$$ \text { The descending order of acidity among the following compounds is : } $$

Choose the correct answer from the options given below :

$$ \text { The strongest conjugate acid will result from : } $$

A D-aldotetrose on oxidation with concentrated $\mathrm{HNO}_3$ resulted in optically inactive dicarboxylic acid. The structure of the D-aldotetrose is :