## Core Concept
The question tests the fundamental differences between *Staphylococcus* and *Streptococcus*, two genera of Gram-positive cocci. The key distinctions lie in their microscopic and macroscopic morphology, as well as certain biochemical properties. Understanding these differences is crucial for clinical microbiology.

## Why the Correct Answer is Right
The correct answer, **C. Coagulase test**, highlights a significant biochemical difference. *Staphylococcus* species, particularly *Staphylococcus aureus*, are often identified by their ability to produce coagulase, an enzyme that converts fibrinogen into fibrin, leading to clot formation. This is a critical test because *S. aureus*, a common pathogen, is typically coagulase-positive, whereas many other staphylococci are coagulase-negative.

## Why Each Wrong Option is Incorrect
- **Option A:** While it's true that *Staphylococcus* and *Streptococcus* can be differentiated based on their colony morphology on agar plates, this option is too vague and does not specify a test or characteristic unique to one genus.
- **Option B:** This option is incorrect because both *Staphylococcus* and *Streptococcus* can exhibit catalase activity, but the key distinction lies in the consistency and type of catalase reaction. *Staphylococci* are generally catalase-positive, whereas *Streptococci* are catalase-negative. However, this is not listed as an option.
- **Option D:** This option is incorrect because, although the cell wall structure and teichoic acids are important for the classification of bacteria, the question seems to focus on more straightforward, commonly used differentiation methods like the coagulase test.

## Clinical Pearl / High-Yield Fact
A high-yield fact to remember is that *Staphylococcus aureus* is coagulase-positive and is a common cause of skin infections, respiratory tract infections, and food poisoning. The coagulase test is a rapid and practical method to differentiate *S. aureus* from other staphylococci.

## Correct Answer: C. Coagulase test.