Answer:
The sentence has two errors, a spelling error mother's and a punctuation error (?)
Step-by-step explanation:
The sentence Which was her mothers favorite vase in all the world should be written as Which was her mother's favorite vase in all the world? The use of a noun with an 's possessive as is shown is the correct way to refer to the subject. besides, it is constructed under an information question structure which demands the use of a question mark at the end.