Sana works out ${\left(\frac{7}{9}\right)}^0\times 4^2$ and writes 0. Which reply is correct?

The zero-exponent law turns the fraction into 1, not into 0 — that is Sana's trap. So the value is $1\times 4^2=1\times 16=16$. Reading $4^2$ as $4\times 2$ gives the other trap, 8.