When Mrs. Lacey smiles at her husband and shakes her head near the end of the story, her smile shows that

she is happy to see him or that she is happy for him or she is happy

Her smile shows that she is surprised by his unusual behavior.

Explanation:

"Stray" by Cynthia Rylant is a story about a stray dog found by the Lacey family on a cold snowy winter day. Doris, the Lacey's daughter wanted to keep the dog as her pet but she also knows that isn't going to happen. Mr. Lacey had hardly enough income to feed the family so keeping a pet was out of the question. Still, Doris tried to ask her parents for any type of indication to show they might allow. The dog had been with them just because the weather was too bad for anyone to venture out of the house.

After so many days of bad weather, when it finally became better, Mr. Lacey went to the pound to drop off the dog. But he returned back with the dog for he found the place awfully bad and unhygienic. He exclaims that " (he) wouldn't leave an ant in that place", thereby allowing Doris to finally have her pet. Mrs. Lacey smiled at her husband at the end of the story for she was completely surprised with the behavior of her husband. For he had commented at the start of the story that he "knows for sure where the pup will be going" suggesting there was no chance of them keeping it when Doris first brought it in the house. This departure of his decision from not keeping it to keeping it is a complete change in his behavior.