Which of the following conditions would tend to make the Hardy-Weinberg equation more accurate for predicting?

Little gene flow with surrounding populations.

Any of the others are likely to cause some sort of selection, or bias, that would not have a random selection type of mating.

The above answerer is correct.  What the question is referring to is that many populations are roughly in genetic equilibrium.  In other words, if you calculate the frequencies of the various alleles at one time, you can predict that the frequencies will be roughly the same in the future.  

However, there are five critieria that a population must have in order to be in genetic equilibrium.  Put another way, there are five things that, if violated, will make a prediction based on the the Hardy-Weinberg equation less accurate.  They are:

1)  A large population size  (violated by option 'A')
2) Random mating  (violated by option 'C')
3) No mutations (violated by option 'E')
4) No migration
5) No natural selection (violated by option 'D')

And so option B, which is basically a more accurate phrasing of the fourth criterion above, is the correct one.