take each of the options and check whether it satisfies the given properties of f
eg option (a) f(x,y) = m+ xy
replace x by x and y by y+k ie f(x, y + k) = m+x(y+k)= m+xy+kx=f(x,y)+kx : satisfies.
note that it also satisfies remaining 2 properties (given)
option b and c doesn't satisfy
hence option a
"You don't have to believe in God, but you should believe in The Book." -Paul Erdős