Positive feedback
a. has an endpoint
b. is common in biological systems.
c. is where a stimulus produces a response which increases that stimulus.
d. is usually associated with regulatory processes.

a. Has an endpoint

c. is where a stimulus produces a response which increases that stimulus.

Explanation:

Positive feedback mechanisms are the one wherein change in the body's physiological conditions is intensified to move the system away from the set point. The processes with endpoints are regulated by positive feedback. Childbirth (birth of a child is the endpoint) is one of the processes regulated by positive feedback.

The start of labor should by culminated with childbirth to ensure the survival of both the mother and child. First contractions in uterus serve as stimuli which in turn trigger the release of oxytocin from posterior pituitary into the blood. The release of oxytocin further intensifies the uterine contractions of smooth muscles and stretching of the cervix to push the baby down the birth canal. The process of uterine stretching and the release of oxytocin to facilitate more contractions continues until the endpoint is reached (childbirth).