An acidic flux among the following is

An acidic flux [eg: SiO₂] is a chemical substance which removes the basic impurities like MnO,FeO,CgO

$$\begin{gathered} \underset{\underset{\underset{\mathrm{Gangue}}{\mathrm{OR}}}{\mathrm{Basic} \mathrm{impurity}}}{\mathrm{CaO}}+\underset{\mathrm{Acid} \mathrm{flux}}{{\mathrm{SiO}}_2}\stackrel{∆}{\to }\underset{\mathrm{Slag}}{{\mathrm{CaSiO}}_3\uparrow } \\ \mathrm{FeO}+{\mathrm{SiO}}_2\stackrel{∆}{\to }\underset{\mathrm{slag}}{{\mathrm{FeSiO}}_3} \end{gathered}$$
the flux combines with the impurity  to form slag
→ MgO, CaO are basic flux