the graph shown in the figure represents a plot of current versus voltage for a given semiconductor. Identify the region if any over which the semiconductor has a (i) negative resistance (ii) where ohm's law is obeyed

Dear student
As current decreases in region BC but there is increase in voltage hence the resistance is negative in region BC 
ohms law is valid for region AB since when we draw  tangent  from A to B it makes approx straight line 

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