The correct answer is D. ornate.
An ornate shawl is one that is decorated with a lot of intricate detail. It is often made with expensive materials and is considered to be a luxury item. In the context of the question, the sister would likely appreciate the ornate shawl as a wedding gift because it is a beautiful and thoughtful gesture.
The other options are not as appropriate for the context of the question. Fatuous means foolish or silly, so it would not be a good choice for a wedding gift. Sober means serious or restrained, so it would not be a very exciting or festive gift. Garnish means to decorate or embellish something, but it is not usually used to describe clothing.
Therefore, the correct answer is D. ornate.