Let ‘I’ be the length of the resistor and ‘a’ be the cross-sectional area of the resistor with resistance R = 1 Ohm.

When a wire is stretched to double its original length,

As a result, length l’ = 2l and cross-sectional area a’ = a / 2

Now we get Resistance (R’) = ρ l’/ a’

R’ = ρ 2l / a / 2

R’ = 4 ρ (1 / a)

R’ = 4R

R’ = 4 × 1

R’ = 4 Ohm