(c) : $a_{1}$ divides $b_{2}$\\ Each element has 2 choices.\\ $\Rightarrow 3 \times 2 = 6$ $a_{2}$ divides $b_{1}$\\ Each element has 2 choices.\\ $\Rightarrow 3 \times 2 = 6$ $\therefore$ Total number of relation is, $6 \times 6 = 36$