The answer is one (yellow), because...
Bandwagon technique is when the marketer persuades by basically saying, "Look at all these people interest in [product name]! You should join because a lot of people have!"
1) because it states that "we couldn't let this weekend pass without saluting the millions who will take part in the annual Make a Difference Day. You can still find a project and join them if you hurry."
As you can see, the key words/phrases are "millions" and "join them".
2) may mention the Difference Day has been going on for 20 years but it is not bandwagon--so basically giving info, not relevant
3) also giving info and also not relevant
4) more persuasion but using empathy and motivation tactics
