**Core Concept**
The Millennium Development Goals (MDGs) were eight international development goals for the year 2015 that had been established following the Millennium Summit of the United Nations in 2000. The MDGs aimed to address extreme poverty, hunger, disease, gender inequality, and access to education, among other issues.

**Why the Correct Answer is Right**
The MDGs targeted reducing child mortality, improving maternal health, combating HIV/AIDS, malaria, and other diseases, ensuring environmental sustainability, and developing a global partnership for development. However, achieving universal primary education was not explicitly stated as a goal, but rather as a target to be achieved by 2015 under the broader goal of achieving universal education. Therefore, the correct answer is not explicitly listed as one of the goals.

**Why Each Wrong Option is Incorrect**
**Option A:** Reducing child mortality and improving maternal health were both targets under the broad goal of reducing child mortality. This option is not the correct answer as it was targeted in the MDGs.
**Option C:** Combating HIV/AIDS, malaria, and other diseases was one of the MDGs. This option is not the correct answer as it was targeted in the MDGs.
**Option D:** Ensuring environmental sustainability was one of the MDGs. This option is not the correct answer as it was targeted in the MDGs.

**Clinical Pearl / High-Yield Fact**
When considering the MDGs, it is essential to note that while achieving universal primary education was not explicitly stated as a goal, it was mentioned as a target under the broader goal of achieving universal education.

**Correct Answer:** D. Ensuring environmental sustainability.