Answer
(c) it is reduced to $\frac{1}{9}$th its original value.
Work Step by Step
$F=k\frac{q_{1}q_{2}}{r^{2}}$
So, if r is tripled, then F will be $\frac{1}{3^{2}}=\frac{1}{9}$th of the original value as evident from the equation above.
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