Step 1: Using the perpendicularity condition.
When two vectors are perpendicular, their dot product is zero. The dot product of \( \vec{A} \) and \( \vec{B} \) is given by:
\[
\vec{A} \cdot \vec{B} = a_1 b_1 + a_2 b_2 = 0
\]
This implies:
\[
a_1 b_1 = -a_2 b_2
\]
Step 2: Solving for the ratio.
Rearranging the equation, we get:
\[
\frac{a_1}{b_2} = -\frac{a_2}{b_1}
\]
Step 3: Conclusion.
Thus, the correct answer is (D), \( \frac{a_1}{b_2} = - \frac{a_2}{b_1} \).