Answer:
d. Since education is vitally important, all Americans must be concerned about learning.
Step-by-step explanation:
A complex sentence is one where the sentence contains both an independent clause and a dependent clause.
An independent clause is a group of words or phrases that make a meaningful and full sentence while a dependent clause cannot stand on its own and make a meaningful sentence.
In a complex sentence, the two types of clauses are joined by a subordinating conjunction which will make the connection between the dependent and the independent clause.
Thus, the sentence
"Since education is vitally important, all Americans must be concerned about learning"
contains both of the clauses.
So, the answer that makes the best complex sentence is option (d).