Number of cache blocks $= 2c$
Number of sets in cache $=\dfrac{2c}{2}=c$ since each set has $2$ blocks. Now, a block of main memory gets mapped to a set (associativity of $2$ just means there are space for $2$ memory blocks in a cache set), and we have $2\,cm$ blocks being mapped to $c$ sets. So, in each set $2m$ different main memory blocks can come and block $k$ of main memory will be mapped to $k \mod c.$
Correct Answer: B.