A transitive verb demands an object, such as in the following case: She is reading a book. The verb to read demands the direct object a book which is why it is transitive.
On the other hand, intransitive verbs do not need an object to complete them, such as in the case: She is sleeping.
In the sentence above, the verb was is a past tense of the verb to be, and it is used as a linking verb in the sentence, connecting the first part with the adjective beautiful. Since there is no object in the sentence, the correct answer is intransitive.