Find the missing number in 1324.

To express the fraction in its lowest form, find the greatest common divisor (GCD) of the numerator and the denominator.
The prime factorization of 84 is $2^2 \times 3 \times 7$.
The prime factorization of 108 is $2^2 \times 3^3$.
The GCD of 84 and 108 is $2^2 \times 3 = 4 \times 3 = 12$.
Divide both the numerator and the denominator by their GCD:

$$\frac{84 \div 12}{108 \div 12} = \frac{7}{9}$$

The lowest form is $\frac{7}{9}$.

The correct option is E.

The final answer is $\boxed{\text{E. } \frac{7}{9}}$.