Correct Option: B

Given that, x(n) = 6 cos		2πn
		4

Here, N =	2π	· m =	2π	· 1 = 4
	Ω		2π
			4

at n = 0

⇒ x(0) = 6 cos		2π	· 0		= 6
		4

at n = 1

⇒ x(0) = 6 cos		2π	· 1		= 6 x 0 = 0
		4

at n = 2

⇒ x(0) = 6 cos		2π	· 2		= - 6
		4

at n = 3

⇒ x(0) = 6 cos		2π	· 3		= 6 x 0 = 0
		4

As we know that power of discrete time periodic signal is given by the relation

	N - 1
P = 1/N	∑	|x(n)|2
	n = 0

	3
= 1/4	∑	|x(n)|2
	n = 0

=	1	[x(0)|2 + |x(1)|2 + |x(2)|2 + |x(3)|2 ]
	4

=	1	[(6)2 + (0)2 + (– 6)2 + (0)2 ]
	4

=	1	[36 + 0 + 36 + 0]
	4

= 72 4 = 18 watts.
Hence, alternative (B) is the correct choice.