What is the Thevenin resistance seen from the terminals

What is the Thevenin resistance seen from the terminals AB of the circuit shown above in the figure?

The given circuit:

The equivalent circuit for calculation of R_th is shown below—

Now, from above figure

1	=	1	+	1	+	1
R_th	=	4	+	6	+	12

1	=	3 + 2 + 1
R_th	=	12

R_th =	12	= 2Ω
	6

Hence alternative (A) is the correct choice.