**Core Concept**
_Rickettsiae_ are obligate intracellular bacteria that typically require a host cell for growth and replication. However, some species have been found to be cultivable in cell-free media, which is a significant exception to the general rule.

**Why the Correct Answer is Right**
_Rickettsia prowazekii_ is the species of rickettsiae that is able to grow in cell-free media. This is due to its ability to undergo a unique phase of growth called the "L-form" in which the bacteria lose their cell wall and can survive and replicate without the need for a host cell. This property makes _Rickettsia prowazekii_ distinct from other obligate intracellular rickettsiae.

**Why Each Wrong Option is Incorrect**
**Option A:** _Rickettsia typhi_ is another species of rickettsiae but it is not able to grow in cell-free media. It requires a host cell for growth and replication.
**Option B:** _Coxiella burnetii_ is a bacterium that can be grown in cell-free media but it is not a rickettsia. It is a distinct genus within the order Rickettsiales.
**Option C:** _Anaplasma phagocytophilum_ is an obligate intracellular bacterium that requires a host cell for growth and replication. It is not able to grow in cell-free media.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the ability of _Rickettsia prowazekii_ to grow in cell-free media is a unique property among rickettsiae and highlights the complex biology of these obligate intracellular pathogens.

**Correct Answer:** D. _Rickettsia prowazekii_