The correct answer is option 'A': 2011-2020.

Explanation:
The United Nations Decade on Biodiversity was designated from 2011 to 2020. This Decade was declared by the United Nations General Assembly (UNGA) in order to raise awareness about the importance of biodiversity conservation and to promote actions to halt its loss.

Now let's break down the answer into headings and key points for a visually appealing and organized response:

1. United Nations Decade on Biodiversity:
- The United Nations Decade on Biodiversity was a 10-year period dedicated to addressing the urgent need for biodiversity conservation.
- It aimed to raise awareness about the significance of biodiversity and the threats it faces.

2. Designation of the Decade:
- The United Nations General Assembly (UNGA) declared the Decade on Biodiversity through a resolution.
- The UNGA resolution was adopted on December 20, 2010.

3. Duration of the Decade:
- The Decade on Biodiversity spanned from 2011 to 2020.
- This 10-year period was chosen to provide a sustained effort to address biodiversity loss and promote conservation.

4. Objectives of the Decade:
- The main objectives of the Decade on Biodiversity were to raise awareness and promote actions to combat biodiversity loss.
- It aimed to enhance understanding of the value of biodiversity and the services it provides to humanity.
- The Decade also sought to encourage governments, organizations, and individuals to take concrete steps towards conserving biodiversity.

5. Significance of the Decade:
- Biodiversity is essential for the functioning of ecosystems and for human well-being.
- However, biodiversity loss has been accelerating at an alarming rate due to factors such as habitat destruction, pollution, climate change, and overexploitation of natural resources.
- The Decade on Biodiversity aimed to reverse this trend by mobilizing global efforts to preserve and restore biodiversity.

In conclusion, the United Nations Decade on Biodiversity was designated from 2011 to 2020 to promote biodiversity conservation and raise awareness about the importance of protecting Earth's natural resources.