What systems lend themselves best to the study of environmental factors that lead to the maintenance ...

What systems lend themselves best to the study of environmental factors that lead to the maintenance of sexual reproduction?
 
  a. cyclical parthenogenetic populations that follow highly predictable seasonal patterns independent of environmental changes
  b. obligate sexual species taxa such as most birds
  c. species that are polymorphic for sexual/asexual reproduction and show the same proportion of sexual reproduction in all environments
  d. populations that respond to environmental changes with a change from asexual to sexual reproduction