Answer:
B.
Step-by-step explanation:
A complex sentence is the sentence that contains one independent and one dependent clause in it. Independent clause is the clause that can express a complete meaning or sense, whereas, a dependent clause is the one that can not stand alone and do no make complete sense.
The given sentence is a complex sentence because it has one independent and one dependent clause. The independent clause in the sentence is:
" she forgot to put her name on it."
Dependent clause is:
"When Sara turned her homework in"
So, the correct answer is option B.