To find the subject of a sentence ask who or what, the sentence is about.
<span>For this sentence it is (a.) Marsupials.</span>
The theme in the Metamorphosis is an irrational or unacceptable event, how Gregor turned into a situation where he can't do anything but to accept the consequences of his actions. It can relate to Kafka's own life where to face the absurd event might be as difficult as Gregor's experience.
There might be many reasons. What an author may right depends on the piece of literature written.
