I would rather choose the third option. I am not suggesting that option "b" is not appropriate. In fact, it's a synonym. Here are both definitions:
bru·tal / 'bru;tl; NAmE / adjective
1 violent and cruel:
a brutal attack / murder / rape / killing
2 direct and clear about sth unpleasant; not thinking of people’s feelings:
With brutal honesty she told him she did not love him.
rig·or·ous / 'rIgJrJs; NAmE / adjective
1 done carefully and with a lot of attention to detail
a rigorous analysis
2 demanding that particular rules, processes, etc. are strictly followed
The work failed to meet their rigorous standards.