The colour of flame depends on: (a) Nature of combustible substance (b) Oxygen…

The colour of a flame is governed by two independent factors: the chemical identity of the substance that is burning, since different atoms and molecules emit light at their own characteristic wavelengths when heated, and the supply of oxygen reaching the flame, since this decides whether combustion is complete (a hotter, cleaner, blue flame) or incomplete (a cooler, luminous, yellow flame produced by glowing unburnt carbon particles).

Applying this here: opening or closing a Bunsen burner's air hole while burning the same gas fuel changes the flame from a cooler, luminous yellow flame (restricted air, incomplete combustion) to a hotter, clean blue flame (ample air, complete combustion) — showing that oxygen supply genuinely changes flame colour for a fixed fuel. Separately, a sodium salt and a copper salt held in the same flame under identical air supply glow with different characteristic colours, yellow versus green, showing that the nature of the substance genuinely changes flame colour too. Since both factors independently and demonstrably change the colour observed, both must be included.

• Restricting the cause to the nature of the combustible substance ignores that oxygen supply on its own (same fuel, more or less air) still changes the flame colour, so substance identity cannot be the sole determinant.

• Restricting the cause to oxygen supply ignores that changing the substance on its own (same air supply, different fuel or salt) still changes the flame colour, so oxygen supply cannot be the sole determinant.

• Pairing the nature of the substance with the temperature of the surroundings wrongly substitutes the ambient temperature of the surrounding air for the flame's own combustion conditions; the temperature of the room does not alter the substance's emission spectrum or the oxygen reaching the flame, so it does not determine flame colour.

Cross-check: this matches the standard classroom demonstration of combustion and flame — the nature of the combustible substance and the oxygen supply are the two recognised determinants of flame colour observed in practice.