Resistance (R) of a copper wire of length l and cross-section A is given by the expression,

If the diameter (radius) is doubled, the new radius r = 0.5 mm = 0.0005 m

=2=3.14×(0.0005)2=0.000000785 2

So, the new resistance will be

Hence, the new resistance will become ¼ times the original resistance.