The one which is a covalent green oil is:

The correct answer is Option (2) → Mn₂O₇

As the oxidation number of a metal increases, the ionic character of its oxide decreases. In \( \ce{Mn2O7} \), 2x + 7(−2) = 0 gives Mn = +7, the highest state; the oxide is covalent, and Mn₂O₇ is a covalent green oil. It is strongly acidic:

    \( \ce{Mn2O7 + H2O -> 2HMnO4} \)

Why the other options are incorrect

• Option (1) CrO₃ — Incorrect. x + 3(−2) = 0 gives Cr = +6; a red crystalline solid with a low melting point, not an oil.

• Option (3) V₂O₅ — Incorrect. 2x + 5(−2) = 0 gives V = +5; an orange solid, amphoteric; used as the catalyst in the Contact process.

• Option (4) CrO — Incorrect. x + (−2) = 0 gives Cr = +2; it is a basic, ionic black solid.

NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… As the oxidation number of a metal increases, ionic character decreases. In the case of Mn, Mn₂O₇ is a covalent green oil. Even CrO₃ and V₂O₅ have low melting points. In these higher oxides, the acidic character is predominant. Thus, Mn₂O₇ gives HMnO₄ and CrO₃ gives H₂CrO₄ and H₂Cr₂O₇. V₂O₅ is, however, amphoteric though mainly acidic …”