Assuming the italicized clause is <em>Since its establishment, </em>the correct answer here is elliptical clause.
Elliptical means that a word which would normally be part of a sentence is excluded, usually for stylistic reasons. Here, the verb is missing, which is why it is elliptical. The question is oddly formulated too - given that there is no verb here, it cannot be a clause, but rather a phrase, but since there is no such option here, then the correct answer is elliptical clause.
Answer and Explanation:
Cleft sentences are used to connect something previously understood to new information or to emphasize something by placing it in a different clause. When we use this type of structure, we are basically dividing a piece of information into two clauses. It is common to use <em>it</em> or <em>wh-</em> words to form the cleft sentence.
1. I don't like a manager who acts like a tyrant. = What I don't like is a manager who acts like a tyrant.
2. We're looking for someone who is a team player. - What we are looking for is someone who is a team player.
3. He would prefer to walk to the theater. - What he would prefer is to walk to the theater.
4. She wrote the most popular mystery novel of the year. - It was she who wrote the most popular mystery novel of the year. / What she wrote was the most popular mystery novel of the year.
5. We saw the most dangerous snake in the world. - What we saw was the most dangerous snake in the world. / It was the most dangerous snake in the world that we saw.
6. She had a terrible case of the flu. - What she had was a terrible case of the flu.
7. I don't understand why someone took my headphones. - What I don't understand is why someone took my headphones.
