Step 1: Understanding asexual reproduction modes.
Different organisms reproduce asexually by specific mechanisms such as budding, regeneration, or gemmule formation.
Step 2: Analysis of each pair.
Ephydatia (a sponge) reproduces by gemmule formation – correct.
Hydra reproduces by budding – correct.
Planarians reproduce by regeneration – correct.
Step 3: Identifying the mismatch.
Ascidians reproduce mainly by budding, not by gemmule formation.
Step 4: Conclusion.
Hence, the mismatched pair is Ascidians – Gemmule formation.