Use the name of the toxin as the clue and confirm by eliminating the rest.
The word aflatoxin is built directly from $Aspergillus$ $flavus$ toxin, which immediately flags the producing organism. Aflatoxins are dangerous mould toxins that grow on poorly stored peanuts and cereals and are well known for damaging the liver and promoting liver cancer. Their recognised producers are A. flavus together with A. parasiticus.
Now rule out the others. $Aspergillus$ $niger$ is the species linked to fungal ear infection rather than aflatoxin production. $Candida$ $albicans$ is a yeast that causes thrush and does not make this toxin. Actinomycetes are filamentous bacteria and are unrelated to aflatoxin.
So the source of aflatoxin is Aspergillus flavus.
\[\boxed{\text{Aspergillus flavus}}\]