The subject is Miss williams
If the letter you are referring to is the one you get when you have your credit card account or bill delivered to your home, then the best answer would be most likely the second one. The letter provides the recipient information about the condition of his own bank account.
The first word in the sentence is a pronoun- not an adjective.
The second option - Australian is in fact an adjective, it gives some information about the terrier.
the second option - children - is a noun, so it is not an adjective.
And the last option is a verb,
So the correct answer is B - this is the only adjective.
