**Core Concept**
The mature cysts of Entamoeba histolytica and Entamoeba coli are two distinct species of amoebic parasites. They have different morphological characteristics, which are essential for their identification. The cysts are the infective forms of these parasites, and their identification is crucial for diagnosing amoebiasis.

**Why the Correct Answer is Right**
Entamoeba histolytica has a distinctive feature of having four nuclei in its mature cyst, with two nuclei being large and two being small. This characteristic is diagnostic for E. histolytica. In contrast, Entamoeba coli has four nuclei as well, but all four nuclei are of equal size. This subtle difference in nuclear size is essential for differentiating these two species.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not specify the correct differentiating feature between the two species.
**Option B:** This option might be incorrect because it could be a characteristic shared by both species, or it might not be a reliable differentiating feature.
**Option C:** This option is likely incorrect because it does not mention the correct differentiating feature of nuclear size between the two species.

**Clinical Pearl / High-Yield Fact**
When differentiating between Entamoeba histolytica and Entamoeba coli, it is essential to note that E. histolytica is a pathogenic species that can cause amoebiasis, while E. coli is a non-pathogenic species. This distinction is crucial for diagnosing and treating amoebiasis.

**Correct Answer: D**