**Core Concept**
The renin-angiotensin-aldosterone system (RAAS) plays a crucial role in regulating blood pressure and fluid balance. Angiotensin II, a potent vasoconstrictor, is a key component of this system, and its levels can increase in response to various physiological and pathological stimuli.

**Why the Correct Answer is Right**
When blood pressure falls, the juxtaglomerular cells in the kidneys detect the decrease in glomerular filtration pressure and release renin. Renin then catalyzes the conversion of angiotensinogen to angiotensin I, which is subsequently converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II causes vasoconstriction, increases blood pressure, and stimulates the release of aldosterone, leading to increased sodium retention and water reabsorption. Thus, decreased blood pressure is a primary stimulus for the release of renin and the subsequent increase in angiotensin II levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased blood volume would typically lead to decreased renin release and decreased angiotensin II levels, as the body attempts to compensate for the increased fluid status.

**Option B:** Raised cardiac output would also lead to decreased renin release and decreased angiotensin II levels, as the body is able to maintain blood pressure without activating the RAAS.

**Option D:** Increased sympathetic tone can stimulate the release of renin, but it is not the primary cause of raised angiotensin levels in response to decreased blood pressure.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to remember that the RAAS is activated in response to decreased blood pressure, not increased blood volume or cardiac output. This can help you eliminate incorrect options and choose the correct answer.

**Correct Answer:** C. Decreased blood pressure