[tex] \frac{a}{b} \: and \: \frac{c}{d} \: are \: rational \: expressions. [/tex]
[tex] \frac{ \frac{a}{b} }{ \frac{c}{d} } = \frac{a}{b} \div \frac{c}{d} = \frac{a}{b} \times \frac{d}{c} = \frac{ad}{bc} [/tex]
So first two options are incorrect.
[tex] \frac{a}{b} \times \frac{c}{d} = \frac{ac}{bd} [/tex]
So third option is wrong and 4th option is right.
Answer is the last one.
