Answer: It was 2015 when Sidra joined university.
Explanation:
A complex sentence has an independent clause, meaning that it can stand alone as a sentence, and one or more dependent clauses, which, despite having a subject and a verb, cannot stand alone. These clauses are not equal, as there has to be a cordinating conjunction, such as "after," or "when," that establishes the rank of one or more of the clauses to make them less equal.
In this example:
Independent clause / subordinating conjunction / dependent clause
It was 2015 / when /Sidra joined university.
