mnieman

H0: Time series has unit root and is nonstationary.
If the test stat is significant, you reject the null. If you reject the null, the series is stationary. So C can’t be the answer.
As a side note, the null in the DF test is exception to the norm - nulls for heteroskadisticity and serial correlation are “no heteroskadasticity” and “no serial correlation,” respectively. The DF null is more in line with the regular t-test null. That is, the nulls for DF and t-tests indicate things we want to reject. Whereas nulls for  heteroskadisticity and serial correlation indicate things we want to accept.
Hope this helps.