We are given that:
\[
a : b = 3 : 4 \quad \text{and} \quad b : c = 8 : 9
\]
To find \(a : c\), we need to make the value of \(b\) in both ratios the same. The least common multiple (LCM) of 4 and 8 is 8. So, we multiply the first ratio by 2:
\[
a : b = 6 : 8 \quad \text{and} \quad b : c = 8 : 9
\]
Now, we can combine the ratios:
\[
a : c = 6 : 9 = 2 : 3
\]
Thus, the correct answer is \(a : c = 2 : 3\).