Answer:
B. When the biology class identified a new species of butterfly, they took careful notes. They also took detailed pictures.
Step-by-step explanation:
Run-on sentences are when two or more independent clauses are improperly connected. This means that the independent clauses are joined in such a way that the sentence seems awkward and incorrect.
In the given sentence, the first part is a dependent clause as it begins with a subordinating conjunction "when". The second and third parts of the sentence are independent clauses.
Now, a dependent clause requires an independent clause to be a complete sentence so, the dependent clause and the second sentence will be joined together with the use of a comma in between. Then, the third sentence will be a separate sentence of its own.
So, the correct answer is option B.