C is the best answer according to the answer key. Yes. However, may I choose B too?. No. It doesn't have the required sense of 'right now' that 'just' suggests. And, if D becomes "has proven", does that sound ok as well? No. It doesn't have the required sense of 'right no
Quite contrary to Clive's opinion, I think B is a more common answer as this statement simply states an observation which generally calls for simple present tense. For instance: