Which of the following plants is primarily used for its ornamental flowers?

Clematis x jackmanii is celebrated primarily for its striking ornamental flowers. This cultivar is part of the clematis family, known for producing an abundance of large, vibrant blooms in various hues and patterns. These flowers not only enhance the aesthetics of gardens and landscapes but also attract pollinators, making them a popular choice among gardeners seeking to add both beauty and ecological value to their planting schemes.

In contrast, the other plants have different primary uses. Campsis radicans, commonly known as trumpet vine, is recognized more for its vigorous growth and ability to provide screening and habitat than for its ornamental qualities. Trachycarpus fortunei, or the windmill palm, is primarily valued for its attractive foliage and hardy nature rather than flowering characteristics. Viburnum awabuki 'Chindo' is primarily grown for its foliage and fruit rather than showy flowers. Therefore, the focus on ornamental flowers clearly aligns with Clematis x jackmanii, making it the standout choice in this question.