Answer:

Solenoidal: A vector $\overline F$ is called as solonoidal if divergence of $\overline F$ is zero. i.e. $\nabla.\overline F=0$ 

We have given

$\overline{F}= \dfrac {{\overline {a}} {\times {\overline {r}}}} {r^n} \cdots\cdots(1)$

We know that,

$\nabla=\big(\dfrac \partial {\partial{x}}\hat i+\dfrac \partial {\partial{y}}\hat j+\dfrac \partial {\partial{z}}\hat k) \cdots\cdots(2)$

$\overline a=a_1 \hat{i}+a_2 \hat{j}+a_3 …