The correct answer is D. a & c only.
Shell-less gastropods, such as sea slugs and nudibranchs, have evolved various defensive adaptations to protect themselves from predators. One of these adaptations is the production of noxious chemicals, which can be toxic or distasteful to predators. Another adaptation is the presence of cnidosacs, which are specialized cells that contain stinging nematocysts from the prey that the gastropod has consumed. When threatened, the gastropod can release these nematocysts to deter predators.
Bright coloration, on the other hand, is not a common defensive adaptation of shell-less gastropods. While some species may have bright colors that serve as a warning to predators that they are toxic or unpalatable, many shell-less gastropods have more subdued coloration for camouflage and to avoid detection by predators.