When Skiff Miller says, An' there's a lot of work in 'm yet. He's good for years to come. An' I do like him, what seems to be going on in his mind?

He is deciding.

Explanation:

The given quote is taken from the short story titled "Brown Wolf" written by Jack London. The story is about the dog-wolf who feels like wild is calling him and that nothing can stop him to chase after his innermost desire of the wild.

In this part of the story, Miller is being questioned that what if the dog wants to go back to where he belongs, where he desires to be. After being questioned, Miller becomes puzzled about what should be done. In the given quote, Miller is trying to decide what should be done. Then finally, he decides to let go off the dog and leave it up to him whether he wants to stay or leave.

Thus, the correct answer is that he is trying to decide.