Which among the following languages is not regular (cannot be defined by a regular expression or finite automaton)?

		a)	L={x | x=ambnck, n, m, k positive integers}
		b)	L={x | x=am(bck)n, n, m, k positive integers}
		c)	L={x | x=(ab4c)n, n a positive integer}
		d)	L={x | x=am(bc6)n, n, m positive integers}