We should do it like this:
Assume we have 4 variable K map. Now if we merge all the 16 cells of this K map we get the function as 1 which is free from all the 4 variables.
Now we can use this to generalize for n variables . If we are able to merge 16 adjacent cells of the K map of n variables hence we will have expression of n-4 variables or we can say n-4 literals.
Hence B should be the correct option.