Answer: Bandwagon.
Explanation: Observing the posters created in other to promote handwashing, one could see people in the picture embracing handwashing protocol, this depicts an appeal to show that majority of people are at risk and are thus embracing the handwashing culture in other to safeguard and prevent the spread of disease. The bandwagon thus means a persuasive technique urging the viewer or reader of the poster to also join in the act of performing what majority of others are doing.