A copper wire having a length of 243 m and a diameter of 4 mm was melted to form a sphere. Find the diameter of the sphere thus formed. 

Volume remains conserved during melting.
Volume before melting = Volume after melting
Before melting, the wire was in cylindrical shape and after melting, it became sphere.
So, Vol. before = $$\pi r^2l$$ = Vol. after = $$\ \frac{\ 4}{3}\pi\ R^3$$, where R is the radius of sphere formed.
$$\left(2\times\ 10^{-3}\right)^2\times\ 243\ =\ \frac{\ 4}{3}R^3$$ 
$$R^3=729\times\ 10^{-6}$$
R=9cm.
So, diameter of the sphere = 18cm.
Option C is the answer.