Correct Option: D

Non-linear phase means group delay varies with varying frequency.
(A)

Phase of	1	= tan–1 (– ω)
	1 + jω

τa(ω) =	– d	tan–1 (– ω)
	jω

=	1
	1 + ω2

which is varying with ω i.e., non-linear.
(B)

Phase of	1	= 2 tan–1 (– ω)
	(1 + jω2)

τb(ω) =	– d	2 tan–1 (– ω)
	dω

=

2

1 + ω2

which is varying with ω i.e., non-linear.

(C)

Phase of	1	= – tan–1(ω) – tan–1	ω
	(1 + jω) (2 + jω)		2

τc(ω) =	– d		– tan–1(– ω) – tan–1		ω
	dω				2

=	1	+	2
	1 + ω2		4 + ω2

which again varies with frequency ω i.e., non-linear.
Hence, alternative (D) is the correct choice.