a
The drift velocity is \(v _{ d }=\frac{ eVt }{2 ml }\)

The relaxation time t changes with change of temperature. Actually it decreases with rise of temperature. Thus at a given temperature drift velocity is directly proportional to the potential difference, inversely proportional to length and is independent of cross-sectional area. So when drift velocity is doubled, \(V\) becomes double and \(l\) is half.