The two strains have visually distinguishable types of complex morphology.

One of the strains must not show complex morphology.
(C
The strains must be identifiable by some trait other than complex morphology.
The student is incorrect.

The statement made by the student is incorrect. The two strains can have visually distinguishable types of complex morphology, and it is not necessary for one of the strains to not show complex morphology. Additionally, the strains can also be identifiable by some other trait, but it does not mean that complex morphology cannot be one of the identifying traits.