He studies penicillin, which kills disease bacteria and cures people so B.
B. Transparent
(Need to add this line so I can post the answer.)
This is an example of a metaphor; a metaphor is a comparison
between to things that don’t seem alike. In this case attired (a synonym of dressed) is
used to compare the state of visible deep mourning of the woman the sentence is
talking about, with the act of wearing the mourning, and of course, you can’t
wear a feeling. The metaphor is used to highlight the fact that her mourning
was so deep that was actually visible, like a dress.