Final answer:
A reward poster is put up to encourage people to look for and return the lost pet.
Step-by-step explanation:
The best explanation for why someone would put up a reward poster for a lost pet is to encourage people to look for and return the pet. By offering a reward, it incentivizes others to actively search for the lost pet and increases the likelihood of its safe return. The reward poster serves as a means of spreading information and increasing awareness about the lost pet, increasing the chances of someone spotting and returning it.