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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.

2 4 2