Assertion: The impurities such as ${\text{SiO}}_2$ present in iron ore is removed by ${\text{CaCO}}_3$.

Reason: $\text{CaO}$ formed by decomposition of ${\text{CaCO}}_3$ reacts with ${\text{SiO}}_2$ to form slag.

The correct option is A Both assertion and reason are correct and the reason is the correct explanation for the assertion.
Limestone $({\text{CaCO}}_3$), is added as part of the feed during smelting of iron ore. It gets decomposed to $\text{CaO}$.
The reaction involved is:

${\mathrm{CaCO}}_3\left(s\right)\to \mathrm{CaO}\left(s\right)+{\mathrm{CO}}_2\left(g\right)$

The $\text{CaO}$ formed acts as a flux which reacts with silica $({\text{SiO}}_2$) to form fusible slag. The reaction involved is:

$\mathrm{CaO}\left(s\right)+{\mathrm{SiO}}_2\left(s\right)\to {\mathrm{CaSiO}}_3\left(l\right)$

(fusible slag)

The slag melts near the zone of fusion and forms a protective liquid layer. This prevents molten iron from getting oxidised.