The property of a conductor which tends to restrict the flow of current through the conductor is called resistance.
SI unit of resistance is ohm ( Ω ). When a potential difference of 1V applied across a wire gives rise to 1A of current through the wire, then the resistance is said to be 1‌ohm (1Ω).
The resistance of conductor depends on length, thickness, nature of material and temperature of conductor.
(i) If the length is doubled, then R is doubled because the resistance of a conductor is directly proportional to length i.e., R∝l.
(ii) Resistance of a conductor is inversely proportional to the square of its diameter or area of cross-section. So, if the radius of conductor is doubled, its resistance becomes one-fourth.
i.e., ‌‌R∝‌

1

A

and
A=πr2
If
r′=2r, then
A′=πr2=π(2r)2=4πr2=4A
If A′=4A, then
R′=ρ‌

l′

A′

‌=ρ‌

l

4A

=‌

1

4

(ρ‌

l

A

)
‌=‌

1

4

R
Therefore, if radius is doubled, the resistance becomes one fourth.