What do you see in​ the pictures? describe in detail. ​​ how do you feel when seeing

When the Great Hall opened in 1897, visitors marvelled at the beauty created by fifty American painters and sculptors.

An introductory subordinate clause is found at the beginning of a sentence, uses a subordinate conjunction, contains a subject and verb, and is dependent on the rest of the sentence. The subordinate conjunction in this clause is "when". It joins the opening of the Great Hall with the visitors reaction. The subject of the subordinate clause is Great Hall, and the verb is opened. In the other options, the subject, verb, or subordinate conjunction is missing.