A metal ‘M’ does not liberate hydrogen from acids, but reacts with oxygen to…

Concept: In the reactivity series of metals, only metals placed ABOVE hydrogen (e.g. Na, Ca, Mg) can displace hydrogen gas from dilute acids; metals placed BELOW hydrogen (Cu, Hg, Ag, Pt, Au) cannot liberate H₂ from acids at all. A second, independent clue is the colour of the oxide product formed when a metal reacts with oxygen: the oxides/peroxides of alkali and alkaline-earth metals are white or pale, in clear contrast to copper's oxide, which is black.

Application: Since M does not liberate hydrogen from acids, M must lie below hydrogen in the reactivity series — this rules out the metals that sit comfortably above hydrogen and react with dilute acids to release H₂ gas. M also forms a black product on reacting with oxygen; heating copper in air/oxygen gives black copper(II) oxide (CuO), matching this second clue exactly.

• Cu: lies below hydrogen, so it does not liberate H₂ from dilute acids; forms black CuO on heating in air — satisfies both clues.

• Mg: lies well above hydrogen, reacts with dilute acids liberating H₂ gas; its oxide MgO is white, not black.

• Ca: lies above hydrogen, reacts with dilute acids (and even cold water) liberating H₂ gas; its oxide CaO is white, not black.

• Na: a highly reactive alkali metal placed well above hydrogen, reacts vigorously with acids liberating H₂ gas; on burning in oxygen it gives a pale/white product (Na₂O, or the peroxide Na₂O₂ in excess oxygen) — never black.

Cross-check: an independent confirmation is that copper does not dissolve in dilute HCl or dilute H₂SO₄ (no gas is evolved on contact), unlike the other three metals which fizz vigorously on contact with dilute acid — consistent with copper being placed below hydrogen in the reactivity series.

Hence M = Cu.