As the length of wire gets doubled, the cross-sectional area will become half of its previous value because volume of wire remains constant. Hence, we can see that the new resistance is four times the previous resistance.

What happens when a wire is stretched to double its length?

When the wire is stretched to double the length , the area of cross section gets reduced to half. So when the wire is stretched, the resistance multiplies by four times. Hope you understood.

When the wire is stretched to double its length the longitudinal strain produced in it is?

Longitudinal Strain is ratio of change in length to original length. Wire is stretched to double it's original length. Hence change in length = 2×L - L (where L is original length. ) Therefore change in length is L.