Which expression is the correct expansion of ${\left(\frac{x}{3}\right)}^4$?

Power of a quotient: numerator and denominator each take the power, so ${\left(\frac{x}{3}\right)}^4=\frac{x^4}{3^4}=\frac{x^4}{81}$. Multiplying $3\times 4$ gives the trap 12, and leaving the denominator untouched gives $\frac{x^4}{3}$.