Final answer:
The correct option is 'Ever since the Copenhagen Summit (COP 15) last December, climate change has become the trendiest catchphrase.'
Step-by-step explanation:
The correct option that turns the fragment into a complete sentence is: Ever since the Copenhagen Summit (COP 15) last December, climate change has become the trendiest catchphrase.
In this option, the subject 'climate change' is clearly stated, making it a complete sentence. The other options either lack a subject or do not form a complete sentence.
Learn more about Forming complete sentences