Final answer:
C. compound-complex sentence. The structure of the sentence 'Today when I went into work, I was very tired, and I wanted to go back to bed' is a compound-complex sentence.
Step-by-step explanation:
The structure of the sentence 'Today when I went into work, I was very tired, and I wanted to go back to bed' is a compound-complex sentence.
A compound-complex sentence contains two or more main clauses and one or more subordinate clauses. In this sentence, 'Today when I went into work' is a subordinate clause, 'I was very tired' and 'I wanted to go back to bed' are main clauses.
Keywords: compound-complex sentence, main clauses, subordinate clauses