Final answer:
The given sentence is a complex sentence as it contains an independent clause and a dependent clause that provides a reason for the action in the independent clause.
Step-by-step explanation:
The sentence: Marta will not be at soccer practice today because she was stung by a bee on her way home from school is a complex sentence. This is because it consists of an independent clause, Marta will not be at soccer practice today, which can stand alone as a sentence, and one dependent clause, because she was stung by a bee on her way home from school, which cannot stand on its own and is providing a reason for the action described in the independent clause.