System of special ductless glands to coordinate various functions is ____.

The correct option is D

Endocrine system

The secretory glands are of two main types, (i) exocrine glands and (ii) endocrine or ductless glands. The organized system of these glands is called endocrine system. Endocrine glands secrete active substances called hormones. Hormones are informational molecules. They are secreted in response to changes in the environment inside or outside the body. They are secreted into the blood, which distributes it all over the body, especially to their target organs and tissues.