L-isomer of a compound 'A'  $\left({\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_4\right)$ gives a positive test with ${\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ Treatment of  ‘A’ with acetic anhydride yields triacetate derivative. Compound ‘A’ produces an optically active compounds (B) and an optically inactive compound (C) on treatment with bromine water and HNO₃ respectively. Compound (A) is: