Question

# The oxide which can be reduced by hydrogen is ________.A.$N{a_2}O$ B.$K$ C.${K_2}O$ D.$CuO$

Hint: Metal Oxides of metals which are more reactive than hydrogen cannot be displaced by Hydrogen.
So, you can look at the activity series for help(to know the power of each element).

First let us have a look over the reactivity series of the metals. As the power of any metal to reduce other metal’s oxide is dependent on its position in the reactivity series (Arrangement of metals in descending order of their reactivities).
Elements
Lithium
Potassium
Barium
Sodium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Nickel
Tin
Hydrogen
Copper
Tungsten
Mercury
Silver
Gold
Platinum
For example-
Magnesium can reduce the oxide of aluminium since aluminium is less reactive as shown above in the series.
Here,
We are given 4 options and we can see from the given reactivity series that the Potassium($K$ ), Sodium($Na$) and Calcium($Ca$ ) are above Hydrogen.
Whereas Copper($Cu$) is below Hydrogen.
So definitely Hydrogen cannot reduce the oxides of potassium, sodium and calcium.
But with copper it gives the following reaction:
$CuO + {H_2} \to {H_2}O + Cu$
So our answer is option D.

Note:
Reactivity series may not be provided to the students during their exams. So they should learn the sequence. Metals which are present at the top of the reactivity series are also known as powerful reducing agents. Aside from giving knowledge into the properties and reactivities of the metals, the reactivity arrangement has a few other significant applications. For instance, the result of the responses among metals and water, metals and acids, and single relocation responses between metals can be anticipated with the assistance of the movement arrangement.