Complex organic compound (e.g. glucose)
Freezing is the process that could cause ice to form a thin layer over a lake.
Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point.
Positive Feedback: Meaning that the stimulation causes more of the hormone to either produce milk or cause uterine contractions during childbirth to thus speed up the process.
If it were negative feedback it would be an on-off process like the release of insulin and glucagon: insulin when blood sugar is high to lower it, glucagon to heighten the blood sugar when it gets low. And since this isn't an on and off again process, the answer to your question has to be positive feedback
