I have to go with B. Here are two possibilities:
- Neutron Bomb.
There are existing ideas on how to kill people but leave structures untouched, one is a neutron bomb. As long as the neutrons are low enough energy not to induce fission but high enough to break chemical bonds then a neutron bomb will kill all the people and leave the structure both unharmed and non-radioactive.
- Cosmic rays.
Another possibility is cosmic rays. Most cosmic rays consist of ionized hydrogen, i.e. a proton without an electron, giving them a positive charge. Whenever a charged particle is accelerated it will emit radiation (see Bremsstrahlung). This is where the majority of radiation exposure to the crew of modern day space ships comes from. Essentially when a proton hits an atom in the hull of the vessel it is slowed. This acceleration causes it to emit photons. Since the cosmic rays are high energy the photons they emit are well into the uv and x-ray range. These photons then flood the craft.
Luckily there is not enough cosmic rays to immediately cook our astronauts and earths magnetic field protects us for the most part. Furthermore, magnetic shields for cosmic ray have been proposed. However, a weapon could be designed which uses a magnetic field to gently redirect cosmic rays from a large area of space and focus them onto a ship, overwhelming the shielding. The occupants would then find themselves inside an x-ray oven. (side note: no amount of magnetic shielding would protect against neutrons as they have no charge.)
One short-coming of this is that blasting a ship with high energy charged particles may damage some of the electronics (Voyager 2 suffered a malfunction due to cosmic rays). You you may be left with a giant metal casket full of dead marines floating in space.