What is the source of $L$-Lactic acid from the following?

$L$-isomer of tetrose $X$ $(C_4H_8O_4)$ gives positive Schiff's test and has two chiral carbons. On acetylation,$X$ yields triacetate. $X$ also undergoes the following reactions:
$X$ $\xrightarrow{HNO_3} A$ $\xrightarrow{NaBH_4} \underset{\text{Chiral compound}}{B}$
$X$ is $.......$.

Which of the following reduces Tollens' reagent?

Glucose has which functional group$(s)$?

The letter '$D$' in $D$-glucose signifies

To become a carbohydrate,a compound must contain at least