Answer:
Al
Step-by-step explanation:
In order for a contract to be formed, both parties must reach an agreement (usually through offer and an acceptance of terms). The offer made by Greta was a $50 reward for her missing dogs, Al and Ben agree to the contract by searching for the dogs. Since Al finds and returns one of the Dogs, Greta has an obligation to pay Al. As for Ben, since he failed to meet the terms, he should not be paid. On the other hand, since Charlie never saw the posters, there was no contract formed because he could not possibly agree to terms he was not aware, and Greta has no legal obligation to pay Charlie.
Therefore, Greta has a legal obligation to pay Al.