The answer should be G.
F. is incorrect because the microorganisms wouldn't decrease on large fish since that would mean the population of cleaner fish would increase and that won't happen if there are these look-alike fish swimming around. The larger fish would be afraid to let these cleaner fish get near them since they would assume it's the other species of fish.
H. is incorrect as well and it's the same reason as shown in F.
J. is incorrect because there's a chance that the cleaners can either be eaten by these look-alike or they just won't breed in general since they both have very different behaviors in the environment.
Hope this helps!
