For similar triangles, the ratio of their areas is the square of the ratio of their corresponding sides (including heights).
Given the heights of two similar triangles are 3 cm and 4 cm, the ratio of the areas of the triangles will be:
\[
\text{Ratio of areas} = \left(\frac{3}{4}\right)^2 = \frac{9}{16}
\]
Step 2: Conclusion.
Thus, the ratio of the areas is $9 : 16$. So, the correct answer is (A).