Is that 1 ohm ?
Since points C and D are at the same potential (0 or ground), they can be joined together as a single point (junction).
now between A and B there are 6 ohms, and 3 ohms in parallel resulting in 2 ohms effectively ( 6*3/(6+3) ).
This 2 ohms is now connected in parallel with a series combination of 1 ohm (AC) and 1 ohm (DB), ie., 1+1 = 2 ohms.
So effective resistance in this configuration is two 2 ohms in parallel..hence 1 ohm.