What improvement occurs when dark blue text on a black background is given a white outline?

Correct answer: Boundary contrast increases and readability improves

The governing concept is value and edge contrast. Dark blue and black have relatively close values, so the letter boundaries can disappear against the background. A white outline inserts a much lighter border around each letter, sharply separating text from the dark field and improving legibility. It does not make the values similar, create a complementary colour pair, or change size contrast. Thus option B is correct.