Chọn A.

- Phản ứng :  ${{C}_{n}}{{H}_{2n}}{{O}_{2}}+\frac{3n-2}{2}{{O}_{2}}\xrightarrow{{}}nC{{O}_{2}}+n{{H}_{2}}O$

           mol :          1         (1,5n – 1)              n

với ${{n}_{{{O}_{2}}}}=1,25{{n}_{C{{O}_{2}}}}\to 1,5n-1=1,25n\to n=4$. Vậy X là C₄H₈O₂ có 4 đồng phân cấu tạo là:

$HC\text{OOC}{{\text{H}}_{\text{2}}}\text{C}{{\text{H}}_{\text{2}}}\text{C}{{\text{H}}_{\text{3}}}$ ; $HC\text{OOCH(C}{{\text{H}}_{\text{3}}}\text{)C}{{\text{H}}_{\text{3}}}$ ; $C{{H}_{3}}\text{COOC}{{\text{H}}_{\text{2}}}\text{C}{{\text{H}}_{\text{3}}}$ ; $C{{H}_{3}}C{{H}_{2}}\text{COOC}{{\text{H}}_{3}}$