**Core Concept**
The question requires the identification of a Gram-positive coccus that is resistant to penicillin, highlighting the importance of antibiotic resistance in clinical microbiology. The isolate's Gram-positive nature and coccus morphology point towards the genus *Staphylococcus*, which includes both coagulase-positive and coagulase-negative species.

**Why the Correct Answer is Right**
The isolate's resistance to penicillin and its Gram-positive, coccus morphology are characteristic features of *Staphylococcus aureus*. This species is notorious for its ability to develop resistance to multiple antibiotics, including penicillin, through various mechanisms such as the production of beta-lactamases. The coagulase test, which detects the presence of the enzyme coagulase, can help distinguish *S. aureus* from other staphylococcal species. In this case, the isolate's resistance to penicillin and its Gram-positive, coccus morphology make *S. aureus* the most likely identification.

**Why Each Wrong Option is Incorrect**
**Option A:** The description does not match any other species of *Staphylococcus*, and the resistance pattern is more characteristic of *S. aureus*.
**Option B:** *Enterococcus faecalis* is a Gram-positive coccus, but it is typically associated with vancomycin resistance, not penicillin resistance.
**Option C:** *Streptococcus pneumoniae* is a Gram-positive coccus, but it is typically sensitive to penicillin and is not associated with the same level of antibiotic resistance as *S. aureus*.

**Clinical Pearl / High-Yield Fact**
*Staphylococcus aureus* is a significant pathogen in healthcare settings, particularly in patients with indwelling devices or those who have undergone recent surgery. Its ability to develop resistance to multiple antibiotics makes it a challenging organism to treat, and its identification requires a high index of suspicion.

**✓ Correct Answer: D. Staphylococcus aureus**