The correct answer to this question is Give them to Lennie. George felt it was a good idea to give Lennie the puppy because it is better for him to play with a puppy than it is for him to play with a mice. He also thought the puppy would keep Lennie out of trouble.
Though not explicit stated, Lennie does suffer from a mental illness. This causes him to make decisions and do things that are not the best. For Slim, he was happy to be able to get rid of the puppies quickly to George. This question is related to the book Of Mice and Men.