Although 90 percent of the population believes

let us try to simplify the argument --

Premise
Only 20% know enough about DNA to understand a news story about DNA.

this is the same as saying 80% do not know enough about DNA to understand a news story about DNA.

the argument then is --

80% do not know enough about DNA --> 80% do not know enough about medical concepts to make well-informed personal medical choices or to make good public policy decisions about health care

Note that there is a "jump/gap" in the argument -- knowledge about DNA is equated to making well-informed medical choices or making good public policy decisions about health care.

Which answer addresses this? Option D

let us take a look at other answer options --

Option A - Incorrect

The argument makes the assumption that understanding of DNA is required to make well-informed medical choices and public policy decisions on health care. But this does not mean that understanding of DNA is a guarantee to making well informed personal medical choices. There might be other things that are required, in addition to an understanding of DNA, to make well informed personal medical choices.
Since, option A is not required to draw the conclusion, this is incorrect.

Option B - Incorrect

the argument more or less does demonstrate that 80% of people are not well informed and must be informed to make good public policy decisions on health care.

Moreover, this -- "well informed about health care for good public policy decisions about health care to be made". The argument talks about DNA/medical concepts there, not "health care".

Option C - Incorrect

the conclusion talks about "make well-informed personal medical choices OR to make good public policy decisions about health care".
Nowhere can we can infer that one is required for the other.

Since, this is not required for the argument, this is not a flaw.

Option E - Incorrect

But this is not a flaw in the argument but merely an inference that can be drawn.