**Core Concept**
Staphylococcal bacteria are a type of **Gram-positive** cocci, typically found in clusters. They are common inhabitants of the human skin and mucous membranes. Staphylococcal species can be pathogenic, causing a range of diseases.

**Why the Correct Answer is Right**
The correct classification of Staphylococcal bacteria is based on their **morphological characteristics**, specifically their arrangement in clusters. This is due to the way they divide, which is in multiple planes, resulting in a grape-like cluster appearance under the microscope.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Staphylococcal bacteria are not typically found in chains.
**Option B:** Incorrect because while some species can form biofilms, this is not a defining characteristic of their arrangement.
**Option C:** Incorrect as they do not typically have a diplococci arrangement.

**Clinical Pearl / High-Yield Fact**
Staphylococcal infections can range from mild skin infections to severe diseases like bacteremia and endocarditis, emphasizing the importance of identifying and managing these infections promptly.

**Correct Answer:** D. Gram-positive cocci in clusters.