The correct answer is D. A Wikipedia article about global warming
Explanation:
Background information refers to the basic information you need to know about a topic to understand the basics of it. Additionally, when you look for a source to find background information it is important the sources focus on the topic you are looking for. According to this, the best source about global warming is "A Wikipedia article about global warming" because this focuses on the topic "Global warming" while others such as "gas emissions", "rising oceans" or "climate change" are only related topics to the topic "Global warming" but might not provide you basic information about global warming itself.
