I would say it's A because it sounds like the most important one.
Just like other cells, the part of the bacterium that is able to interact with its outside environment is its cell membrane. Generally, the main function of the cell membrane is for barrier and protection. This decides which molecules will be allowed to permeate or not. Hope this helps.
D: Where two continental plates are colliding, creating a convergent boundary. One plate slips under the other because of convection currents in the mantle.
