Final answer:
Examples of effectors include a) skeletal muscle and d) endocrine gland. Hence, a) and d) are correct.
Step-by-step explanation:
The effector organ is the organ or structure in the body that carries out the response generated by the nervous system. In the given options, examples of effectors include skeletal muscle, which contracts to move the skeleton, and endocrine gland, which produces and secretes hormones.
Cardiac muscle and cranial bone are not examples of effectors as they do not carry out responses generated by the nervous system.