**Core Concept**
Staphylococci and streptococci are both Gram-positive cocci, but they can be differentiated by their ability to produce catalase, an enzyme that breaks down hydrogen peroxide. This characteristic distinguishes staphylococci from streptococci.

**Why the Correct Answer is Right**
The catalase test is a simple biochemical test used to differentiate between staphylococci and streptococci. Staphylococci produce catalase, which breaks down hydrogen peroxide into water and oxygen, resulting in the formation of bubbles. This reaction is known as the catalase reaction. On the other hand, streptococci do not produce catalase, and therefore, the hydrogen peroxide remains undecomposed. This test is a crucial tool in the identification of these two types of bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** The coagulase test is used to differentiate between Staphylococcus aureus (coagulase-positive) and other staphylococci (coagulase-negative). It is not used to differentiate between staphylococci and streptococci.
**Option C:** Phosphatase is an enzyme that is not used to differentiate between staphylococci and streptococci. While some bacteria may produce phosphatase, it is not a characteristic used to distinguish between these two types of bacteria.
**Option D:** This option is incorrect because staphylococci and streptococci are both Gram-positive, not Gram-negative.

**Clinical Pearl / High-Yield Fact**
The catalase test is a simple and rapid method for distinguishing between staphylococci and streptococci. It is an essential tool in the clinical laboratory for identifying these bacteria, which are commonly isolated from various infections.

**✓ Correct Answer: B. Catalase test**