The following sentence contains an adverb clause.
This sentence contains two clauses - an independent clause (We moved) and a dependent/subordinate clause (because our house was too small). Given that the second sentence gives us a reason why they moved, it acts as an adverb, which is why it is an adverbial clause.
I think the answer is either A or C. I would go with A! Hope this helps :)
Answer:
bad news was told by her
a letter was written by her