Answer:
b) The population must be small
Step-by-step explanation:
For a population to be in Hardy Weinberg equilibrium, no evolutionary forces must occur (mutation, migration, selection, genetic drift).
For that reason, a very important condition for equilibrium is that populations have infinite size. This makes the allele frequencies variations caused by chance (genetic drift) to be negligible.
If the population's size were small, the genetic drift effects would be too great and the allele frequencies between generations would not remain constant.