The conditions would be low-levels of lactose and high levels of the disaccharide containing fucose. This would give bacteria with the mutant enzyme a selective advantage when the main food source was the non-lactose disaccharide.
Work Step by Step
Natural selection requires that having a trait, here, the possession of an enzyme to digest a fucose-containing disaccharide, which improves survival and reproduction. This is because it is the main food source--bacteria able to use it would grow and reproduce much faster than those relying on lactose.