The answer is A. Processed foods should be avoided because they can lead to serious health problems
Explanation:
Complex sentences are the result of combining an independent clause and a dependent clause. An independent clause can stand by itself and includes a subject and verb; on the other hand, a clause is dependent if it relies on others to be complete.
From the options given the only one that includes these two types of clauses is option A as "Processed foods should be avoided" is independent and "they can lead to serious health problems" is dependent considering without the first sentence it is not possible to know that "they" refers to. Additionally, as in most complex sentences, the two clauses are linked by subordinating conjunction that in this case is "because" that links the two clauses but also shows the dependence relationship.
