There are two points to note from the professor's argument:
First born + male child is first in line to inherit the family's estate.
The student misinterprets that only first born male child will inherit the estate and there is no other exception such as women or only surviving child or both inheriting the estate. From the example he posits, it is clear that he thinks daughter's were not given the estate or only son could inherit the estate.
Next, we choose an option in line with this argument.
(A) Only men can father male children.- This is talking about men's ability to raise/father male children and not about inheritance of family's estate by first born male children. Irrelevant.
(B) A daughter cannot be a firstborn child.- Irrelevant.
(C) Only sons can inherit the family estate.- Correct, as mentioned above.
(D) Illegitimate children cannot inherit their father’s property.- No discussion about illegitimate children as both the professor and student talks about inheritance in a marriage setup, which is considered legitimate.
(E) A woman cannot inherit her mother’s property.- Scope is narrow. The text talks about family's estate which may or may not include mother's property. The student talks about Father's property.
Hence, option C.