Answer
a) Mixture
b) Compound
c) Mixture
d) Mixture
Work Step by Step
a) Air is a mixture of many compounds, predominantly nitrogen ($N_2$) and oxygen ($O_2$).
b) Mineral form of Calcium Fluoride ($CaF_2$).
c) Brass is a metal alloy of copper (Cu) and zinc (Zn); therefore, it is a mixture.
d) 18-carat gold is an alloy of gold: 18 out of 24 parts in mass with the rest being impurities and other metals. Therefore, it's a mixture.
