Answer:
It is a sentence fragment.
Explanation:
A sentence fragment is a group of words that seems a sentence because they begin with a capital letter and end with a punctuation mark like a period, but that, in reality, is not a sentence because it does not express a complete thought.
“Which was her mother’s favorite vase in all the world” is an example of sentence fragment because it begins with a capital letter and end with a period like sentences do, but it does not express a complete thought. This fragment, instead, should be part of a larger sentence.
