In 1941, Beadle and Tatum studied a bread mold, Neurospora crassa. Neurospora can be cultured together with sugar, inorganic salts and the vitamin biotin. This fungus has a short life cycle, and reproduces sexually and replicates asexually—that is, sexual reproduction gives rise to spores. In addition, Neurospora possesses only one set of unpaired chromosomes, so that any mutation is immediately expressed.
Therefore, the correct answer is option A.