I think A\B is an array of integers which is present in A but not in B.
To solve the given problem run the following code
Take another array C[n-1]
parameters i,j,k
i =0,j =0,k=0
A[ n] = ∞
and B[m] = ∞
while(i!=n)
if (A[i] < B[j] )
C[k] = A[i]
i = i+1
k = k+1
end if
if (A[i] ==B[j])
i = i+1
end if
if (A[i] > B[j])
j=j+1
end if
end while
So, the required array A\B = { C[0],C[2],..........,C[k-1]}
If you see the above code carefully it is a little modulation of merge algorithm of merge sort.
So, the time complexity is O (m+n)
which is option C.