Die Politik ist _____ sinnvoll.
Do both (a) and (b) fit into the blank, and would the meaning be "The policy is very sensible" and "The policy is not very sensible", respectively?
You can't use (a) for the blank. You might be able to use (b) however. If you want to use the sentence as it is, you need to use "sehr" for the positive connotation.
However, that sounds like a literal translation for the sentence. Depending on the context I would translate it differently. An example would be "Die Politik ist vernünftig." bzw. "Die Politik ist nicht vernünftig."
The problem with your example is, that you can't use "viel" with any adjective. Instead you use "sehr". So in your example it would be:
You use "viel" with Substantives, as in: