In French you have two possibilities to form the superlative, which differ by the placement of the adjective:

la plus belle robe -- la robe la plus belle

The definite article is repeated in the second case. If you want to know why, I don't think there even is a "why" for that except that's how it is.

@Chris - but this second case is not mentioned in the lesson, is it? Would be great to see more examples.

I think it is discussed in this lesson: Forming the superlative of French adjectives in complex cases

Then perhaps that question should be with that explanation rather than with this lesson.  It is odd to have no explanation for the answer.

Having just reviewed this lesson, I find the explanation not satisfactory (sorry!). The lesson isn't about forming superlatives with complex cases. The test question (rather, the choice of answers) is very misleading as it stands.