There are three ways a sentence can be written-
1) Simple sentence- A simple sentence is made up of only one clause. It is made up of a subject, verb and a completed thought.
2) Compound Sentence- A compound sentence consists of two or main clauses which are joined using a coordinating conjunction ( for, and, nor, but, or, yet, so).
3) Complex Sentence - A complex sentence consists of one independent clause and one or more dependent clause. A dependent clause cannot stand alone as a sentence. Therefore, subordinating conjunctions are used before a dependent clause. Some of the most common subordinating conjunctions include after, although, as ,before,while, wherever, whereas,unless, until,though, when, since, even though,if, since, because.
The correct answer is D)The thieves are believed to have got in through the kitchen window..
Option D is correct as the given sentence is a complex sentence and to convert it into a simple sentence, the dependent clause is contracted to a word or phrase (The thieves are believed to have got in).
The other options are wrong as the meaning is changed, the word believed is missing, and have is not included.