1. I agree with James. A, B and C are all correct. Don't let anyone try to convince you otherwise. This ill-conceived 'test' was written either by a non-native who didn't realise that it had three possible answers, or a native who didn't notice. However, if you have to give answer only, I suspect that it is supposed to be B - the 'must' of assumption/supposition. But you can't discount either A or C. A reputable exam board would never present its students with such a dreadful test. 2. It's correct, and as James says, you need the 'to'. Otherwise, you could use a gerund '..is raising'.

1. Ah, the dreaded multiple choice test. I would take any of A, B or C myself. Good luck! 2.Yes. "vulnerability" means susceptibility or openness to harm here. Either use "is to raise" or "is raising" if you object to "to" for some reason.

1. A 2. Correct. Sinonyms to vulnerability - assailable, victimizable, expugnable. No, you can not.