Not supported. This answer choice suggests that the only reason for ventilating a house is to avoid the risk posed by these toxins. This is never stated or implied by the stimulus. Just because one side-effect of ventilating a house is to get rid of these specific toxins doesn't mean there can't be other reasons to ventilate a house, such as getting rid of unpleasant smells, etc.

Not supported. The stimulus explicitly tells us that plants do effectively remove some household toxins from the air, and cites a study where plants "eliminated" formaldehyde from the air supply. Even if not all toxins are necessarily removed, the removal of some toxins will certainly change the concentration of toxins in the household air supply.

Not supported. We're told that plants remove some toxins, but we don't know how many toxins they remove. So we can't say for sure that the air supply will be "safe": there could still be enough toxins left, which the plants couldn't remove, that the air would still be unsafe. (Though it's nitpicking, note that we also don't necessarily know that simply being well-insulated will guarantee that a house is "warm.")

This is supported. We know plants do remove some toxins, and the only specific toxin we know has been shown to be removed by plants is formaldehyde. So this strongly supports the idea that if we put plants in a house that has some formaldehyde in the air, the concentration of formaldehyde in the air will decrease.

Not supported. While the stimulus tells us that plants can reduce the concentration of formaldehyde in the air supply, we don't know whether benzene is a toxin that can also be removed by plants. So we can't say based on the stimulus that the concentration of benzene would decrease.