I did not like any answers.
Is the OA E?

A. brush just twice a day and floss hardly never - hardly never is not correct idiom.
B. brush your teeth twice and never floss - Why do Americans have to brush your teeth?
C. brush just twice but rarely floss at all - The sentence begins with an although; the but is redundant
D. brush twice daily but hardly ever floss = same as above.
E. only brush twice daily and rarely floss at all

finance wrote:Though dentists recommend brushing one's teeth three times a day, many adult Americans
brush just twice a day and floss hardly never.
A. brush just twice a day and floss hardly never
B. brush your teeth twice and never floss
C. brush just twice but rarely floss at all
D. brush twice daily but hardly ever floss
E. only brush twice daily and rarely floss at all

In all option but B, I am seeing redundancy. And, B seems awkward. "your teeth"

Can you please share OA and OE?

OA is E.
A is wrong because hardly never is not idiomatic.
B because of "your"
C,D no need to change direction by using BUT
E remains:)