Final answer:
The slogan is the element that will best help people remember the PSA's message due to its catchiness and memorability.
Step-by-step explanation:
The element that will best help people remember the PSA’s message “Spread Music. Not flu.” is likely the slogan. Slogans are designed to be catchy, concise, and easily memorable. In the realm of public service advertisements, a powerful slogan can resonate with the audience and inspire them to remember and act on the message. While images and animations can certainly contribute to the overall impact of an advertisement, a well-crafted slogan often serves as the verbal anchor that sticks in people's minds long after they have seen the PSA.