Gray Code Conversion
The question is asking you the value of N where the value of Gx = 1 for N+1.

Map the current state N and Next state N+1 for the 4 bit Gray code



Read off the min term for each next case of Gx = 1

G0 = 0.1.6.7.12.13.10.11
G1 = 1.3.2.6.13.15.14.10
G2 = 2.6.7.5.4.12.13.15
G3 = 4.12.13.15.14.10.11

--> Answer = G2 is correct though they have the terms out of order in the question