Correct Answer - D
By placing two wires each of length `2a`, along `y`-axis, complete the square loop in `y-z` plane and in `x-y` plane.
From `P(a,0,a):`
Current in loop `①` is anticlockwise, hence magnetic field at `P` will be towards observer i.e.
`vec(B)_(1)=Bhati`
Current in loop `②` anti clockwise,
`vec(B)_(2)=B hatk`
`vec(B)_(P)=vec(B)_(1)+vec(B)_(2)=B hati+Bhatk`
`hat(B)_(P)=(vec(B)_(P))/(|vec(B)_(P)|)=(Bhati+Bhatk)/(sqrt(2)B)=1/(sqrt(2))(hati+hatk)`