The correct option is A 6.83
All the transistors are in saturation region, thus
ID=μnCox2(WL)3(VGS3–VTN)2
=362×10−6×(27.8)×(2−1)2
ID=0.5mA
Now, since all the transistors are connected in series, thus
ID=μnCox2(WL)1(VGS1–VT)2
0.5×10−3=36×10−62(1.75)(VGS1–VT)2
0.5 mA=36×10−62(1.75)[10–Vx–1]2
Vx=5.01V
ID=μnCox2(WL)2(VGS2–VT)2
0.5×10−3=36×10−62(WL)2(Vx−2−1)2
(WL)2=6.83