To evaluate which pair is an incorrect match, let's analyze the characteristics of each mentioned group in the options.

Option A: Annelids and arthropods - Bilateral symmetry

Annelids (segmented worms) and arthropods (such as insects, spiders, and crustaceans) both exhibit bilateral symmetry. This means they have a body plan that can be divided into symmetrical halves along a single plane. Therefore, this match is correct.

Option B: Sponges - Acoelomates

Sponges are indeed acoelomates. They lack a true body cavity (coelom) and are known for their simple, porous body structure. Hence, this match is also correct.

Option C: Coelenterates and Ctenophores - Radial symmetry

Both coelenterates (such as jellyfish and corals) and ctenophores (comb jellies) possess radial symmetry. Radial symmetry means their body parts are arranged around a central axis. Thus, this match is correct.

Option D: Platyhelminthes - Diploblastic organisation

Platyhelminthes, commonly known as flatworms, actually exhibit triploblastic organisation, meaning their body structure arises from three primary germ layers: the ectoderm, mesoderm, and endoderm. Diploblastic organisms have only two germ layers: the ectoderm and endoderm. Therefore, this match is incorrect.

So, the incorrect match is Option D: Platyhelminthes - Diploblastic organisation.