I don't like these four choices since B and D aren't really mutually exclusive. Something can be both tactics and selfishness. I think that this is both B and D. But if I have to choose, I'll go with B.
I say that this is B because the only reason that Napoleon wants to pretend that he's opposed to the windmill is because he wants to be against Snowball. He wants to have all the power so he wants to oppose everything that Snowball does so that he can try to get Snowball out of power.