**Core Concept:**
The incubation period of an infectious disease refers to the time from infection to the onset of symptoms. In the case of Ebola virus, this is the duration between exposure to the virus and the manifestation of Ebola disease symptoms. Ebola virus is a highly pathogenic RNA virus in the Filoviridae family, causing severe hemorrhagic fever in humans and non-human primates.

**Why the Correct Answer is Right:**
The correct answer is **D** (21 days) because the incubation period of Ebola virus is typically around 21 days, with a range of 2-21 days. This wide range is due to the virus's ability to hide within the body without causing symptoms, known as latency. During this time, the infected person can still transmit the virus to others.

**Why Each Wrong Option is Incorrect:**
A. **2 days (Option A)** is too short, as the virus may take more time to manifest due to latency and immune response.
B. **7 days (Option B)** is also too short, as the range includes longer incubation periods.
C. **21 days (Option C)** is too long, as the actual incubation period is closer to 21 days.

**Clinical Pearl / High-Yield Fact:**
Understanding the incubation period of Ebola virus is crucial for public health measures and contact tracing, as the infected person may not show symptoms but can still transmit the virus to others during the latent period. This emphasizes the importance of isolating and monitoring individuals with Ebola-like symptoms even if they have a long incubation period.