The author assumes that people who cannot understand news stories about DNA are unable to make well-informed personal medical decisions (or that people who can make these decisions must be able to understand news stories about DNA). (A) has this backward. It's commiting the sufficiency-necessity mistake.

The author concludes that 80% of people can’t make good public policy decisions because they can’t understand a news story about DNA, but he never suggests that over 20% of people need to be well informed about health care for good public policy decisions to be made.

The author never suggests a causal connection between making well-informed personal medical choices and making good public policy decisions about health care. Instead, he assumes that one has to understand DNA in order to make these personal and public health decisions.

This describes the author’s unsupported assumption. He claims that 80% of people can’t make good personal medical decisions or public policy decisions because they can’t understand DNA, but he fails to demonstrate that understanding DNA is necessary for making those decisions.

Just because 80% of people don’t understand DNA doesn’t mean they’re not well informed about health care. The author doesn’t argue that not understanding DNA means someone isn’t well informed about health care, but rather that they can't make informed personal medical decisions.