E

We don’t have enough information to make the comparison (E) attempts to draw. The recent tests show that a well-insulated house with houseplants will contain fewer toxins than a well-insulated house without houseplants. However, the recent tests didn’t tell us anything about the relative amount of toxins in well-ventilated houses. Maybe well-ventilated houses have no toxins at all (after all, the passage tells us toxins in houses are “not a problem in well-ventilated houses”), while well-insulated houses have at least some toxins, even with a lot of houseplants.

Why not A?

Hi, we have no reason to think that A is true. We are told that plants can remove some toxins, including formaldehyde. This doesn't show that they can remove benzene as well.