The answer to your question would be that the option that is an example of a complex sentence is the following one: Processsed foods should be avoided because they can lead to serious health problems. That is, the correct option would be C.
A complex sentence has one independent clause and at least one dependent clause. Independent clauses, unlike dependent clauses, can stand alone as a sentence as they represent a complete thought. You can see this in the following examples.
a) Processed foods should be avoided (independent clause)
b) *Because they can lead to serious health problems (dependent clause)
