Not supported. Nothing in the stimulus allows us to conclude that houseplants can specifically remove benzene from the air. The only specific toxin we know they can remove is formaldehyde.

Not supported. The stimulus says that some synthetic products release toxins, though note that the first sentence isn't limited only to synthetic products. This isn't the same thing as claiming that nonsynthetic products do not release toxins.

Not supported. The stimulus doesn't draw any connection between houseplants and trapping heat. Trapping heat is a property of insulation, not of houseplants.

This is correct. When the second sentence says that the release of toxins is not a problem in a well-ventilated house, this suggests that one of the negative effects of poor ventilation will be to keep toxins in the household air supply. And since we know that houseplants remove at least some such toxins, it follows that having houseplants can at least partially counteract some negative effects of poor ventilation.

This doesn't follow. The stimulus doesn't give us any grounds to compare the effects of being well-insulated, but having houseplants, versus being well-ventilated with no houseplants. If anything, the fact that we're specifically told that houseplants remove "some" (but not necessarily all) toxins from the air suggests that a well-insulated house with houseplants might still have more toxins in the air than a well-ventilated house — but the stimulus just doesn't make any comparisons on this point.