You will have to get changed if you want to come out with me. You cannot go in jeans.
Why is the verb get changed the right option?The second sentence shows that the person being referred to is putting on a cloth the speaker considers inappropriate. Therefore the person has to get changed/ wear a different outfit.
The right answer cannot be get dressed because the person being referred to is not undressed.
The verb get changed is considered an idiomatic expression. Idiomatic expressions are words or phrases that have a popular meaning or is well understood in a social-cultural setting.
Learn more about idiomatic expressions here;
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