To simplify the fraction \( \frac{9}{12} \), you need to find the greatest common divisor (GCD) of the numerator (9) and the denominator (12).
The GCD of 9 and 12 is 3.
Now, divide both the numerator and the denominator by their GCD:
\[ \frac{9 \div 3}{12 \div 3} = \frac{3}{4} \]
Therefore, \( \frac{9}{12} \) in lowest terms is \( \frac{3}{4} \).