Answer:
d. visual signal
Explanation:
Changing color by cephalopods ssuch as squid & octopi is a type of visual signal which helps in communication. These creatures are highly social animals and for social behavior communication is a must. So, in order to communicate effectively, they often change the colors and patterns of their skin. Initially this ability to change color was meant for camouflage but later on these animals started using this ability to communicate. The control centre of such visual signals is their brain which can manipulate the chromatophores within their body. They usually use such visual signals during courtship rituals and hunting.
