**Core Concept**
Gram-positive cocci are a class of bacteria characterized by their thick peptidoglycan cell walls, which retain the gram stain. They are typically spherical in shape and include species such as Staphylococcus and Streptococcus.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of gram-positive cocci, specifically their cell wall composition. Gram-positive cocci have a thick layer of peptidoglycan in their cell walls, which is responsible for their gram-positive staining properties. This thick peptidoglycan layer also makes them more susceptible to beta-lactam antibiotics, such as penicillin, which target the bacterial cell wall.

**Why Each Wrong Option is Incorrect**

**Option A:**
This option is incorrect because gram-positive cocci do not typically have a capsule. While some species of gram-positive cocci may have a capsule, it is not a characteristic feature of the group as a whole.

**Option B:**
This option is incorrect because gram-positive cocci are not typically aerobic. While some species of gram-positive cocci may be aerobic, others may be facultative anaerobes or strict anaerobes.

**Option C:**
This option is incorrect because gram-positive cocci do not typically have a flagellum. While some species of gram-positive cocci may have a flagellum, it is not a characteristic feature of the group as a whole.

**Clinical Pearl / High-Yield Fact**
Gram-positive cocci are typically susceptible to beta-lactam antibiotics, such as penicillin, due to their thick peptidoglycan cell walls. This is in contrast to gram-negative cocci, which have a thinner peptidoglycan layer and may be resistant to beta-lactam antibiotics.

**Correct Answer: D.**