Which of the following on hydrolysis forms invert sugar?

The given structure is the enol form of:

The Fischer projection formula of $D-(-)$-fructose is:

How do you explain the presence of all the six carbon atoms in glucose in a straight chain?

$L$-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 $.......$.

Identify the reagent that confirms the presence of five $-OH$ groups in glucose.