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Following the selection criteria which is mentioned in given RA query , we can comment about only about A and B as far as FD is concerned but nothing about C..

Now the condition mentioned is :

If  there are 2 instances of relation R having the same value of the attribute A , then for selection of that tuple it is necessary that for these 2 instances of tuples which have same A attribute value , they contain the different B attribute values

This means those tuples will be selected which have more than 1 B values for a given A attribute value..But in the question it is given that the result of the given query is empty , thus suggesting that no such tuple exists..

In other words , it is clear that for a given value of A we cannot have more than one value of B , thus making the value of B unique for a given instance of A..

So following the definition of FD , the FD which will follow is  : A --> B

Hence A) should be the current answer.. 

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