Let's trace the logic here.
We know some students are taking both French Lit 205 and Bio 218.
We also know that every student in that Bio 218 class is a biology major.
Putting those two facts together, it means there are definitely some biology majors sitting in the French Lit 205 class.
Now, look at the conclusion: "Therefore, some of the students taking French Literature 205 are not French-literature majors."
This final leap in logic only works if you assume that the biology majors we just identified in the French Lit class cannot also be French literature majors.
Think about it: What if you could major in both? If double-majoring was allowed, then those biology majors in the French class might also be French-lit majors. If that were true, the conclusion would completely fall apart.
The argument's conclusion only holds up if it assumes that the two groups (biology majors and French-lit majors) are mutually exclusive.
That's why (E) It is not possible to major in both biology and French literature is the correct assumption. It's the only one that bridges the gap between "There are bio majors in the class" and "Therefore, some in the class are not French-lit majors."
Why the other options are incorrect:
(A) French Literature 205 is a required course for French-literature majors. This tells us about other students in the class (the FLMajors), but it doesn't help us conclude anything about the specific group of students who are BioMajors.
(B) Only biology majors are allowed to take Biology 218. This is just another way of stating Premise 2 ("Every student taking Biology 218... is a biology major"). An assumption must be new information, not a restatement of a premise.
(C) & (D) There are more biology majors than... / There are more French-literature majors than... The relative number of students in each major is irrelevant to the logical structure of the argument, which is about the properties of a specific subgroup.
GmatStuck
At Tromen University this semester, some students taking French Literature 205 are also taking Biology 218. Every student taking Biology 218 at Tromen is a biology major. Therefore, some of the students taking French Literature 205 are not French-literature majors.
The conclusion drawn above follows logically if which one of the following is assumed to be true at Tromen University?
(A) French Literature 205 is a required course for French-literature majors.
(B) Only biology majors are allowed to take Biology 218.
(C) There are more biology majors than there are French-literature majors.
(D) There are more French-literature majors than there are biology majors.
(E) It is not possible to major in both biology and French literature.