**Core Concept**
The Venturi mask is a device used in anesthesia and critical care to provide a precise and controlled flow of oxygen to the patient. It works on the principle of jet mixing, where a high-velocity oxygen jet is directed into a Venturi chamber, which creates a region of low pressure that draws in room air. This mixture of oxygen and room air is then delivered to the patient.

**Why the Correct Answer is Right**
The Venturi mask provides fixed performance oxygen therapy because it delivers a specific concentration of oxygen to the patient, regardless of the patient's respiratory rate or tidal volume. This is achieved by using different Venturi chambers, each with a specific internal diameter, to create a precise ratio of oxygen to room air. The Venturi mask is available in various sizes and can deliver oxygen concentrations ranging from 24% to 60%. The Venturi effect is due to the Bernoulli's principle, which states that the pressure of a fluid decreases as its velocity increases.

**Why Each Wrong Option is Incorrect**
**Option A:** Nasal Cannula delivers a variable concentration of oxygen, depending on the patient's respiratory rate and tidal volume.
**Option C:** O2 by T-piece delivers 100% oxygen, but it does not provide a fixed performance oxygen therapy like the Venturi mask.
**Option D:** SO2 (Sauerbruch's oxygen mask) is not a commonly used term in anesthesia and critical care.

**Clinical Pearl / High-Yield Fact**
The Venturi mask is a safe and effective way to provide a precise concentration of oxygen to patients, especially those with chronic respiratory disease or those who require long-term oxygen therapy. It is also useful in patients who are at risk of oxygen toxicity, as it can deliver a lower concentration of oxygen.

**✓ Correct Answer: B. Venturi mask**