Answer : 3. "threat"
Explanation :

In the context of the sentence, "Global warming or climate change has today become a major threat to mankind," the word "threat" is the most suitable choice. It conveys the idea that global warming poses a significant danger or risk to humanity. The other options, such as "penalty," "punishment," and "endanger," do not accurately capture the sense of imminent harm or danger associated with climate change in this context.