**Core Concept**
Class II circulatory failure, also known as hemorrhagic shock, is a condition characterized by inadequate blood volume, leading to decreased cardiac output and ultimately, insufficient perfusion of vital organs. The classification of hemorrhagic shock is based on the amount of blood loss and its corresponding impact on the circulatory system.

**Why the Correct Answer is Right**
The correct answer, **B. 15-30%**, corresponds to Class II hemorrhagic shock, which is characterized by a moderate decrease in blood volume, resulting in a slight decrease in cardiac output and an increase in heart rate. This decrease in blood volume triggers a compensatory response, including vasoconstriction and increased peripheral resistance, to maintain blood pressure and ensure perfusion of vital organs. The key enzymes and pathways involved in this response include the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system, which release catecholamines to increase heart rate and contractility.

**Why Each Wrong Option is Incorrect**
**Option A:** 40% blood loss is typically associated with Class IV hemorrhagic shock, characterized by severe hypotension, decreased cardiac output, and potentially fatal outcomes.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the classification of hemorrhagic shock is based on the amount of blood loss, not the severity of symptoms. Early recognition and treatment of hemorrhagic shock are critical to prevent progression to more severe classes and improve patient outcomes.

**✓ Correct Answer: B. 15-30%**