In Roald Dahl's "Lamb to the Slaughter" Mary kills Patrick because... (see options)

Choice D.  Mary is heartbroken that Patrick would leave her.  Of the four choices that are listed there, that is the best choice.  

The opening paragraphs of the story clearly show readers that Mary is deeply in love with Patrick.  When the story begins, readers are told that Mary is patiently waiting in the front room for her husband to get home.  She is not only waiting though, she is excitedly waiting.  

Now and again she would glance up at the clock, but without anxiety, merely to please herself with the thought that each minute gone by made it nearer the time when he would come.

The above lines say that Mary kept checking the clock in order to know that each minute passing was another minute closer to seeing Patrick.  That's something my kids do when grandma and grandpa are coming over.  They constantly ask me what time it is or "how much loooonger."  They can hardly wait because they are so excited, and they keep clock checking to make sure that time is indeed passing.  

Once Patrick arrives at home, Mary flutters about the house making sure that Patrick merely has to sit down and enjoy his rest.  She gets up to greet him, she gets him a drink, and she is happy to merely sit quietly next to him.  

For her, this was always a blissful time of day. She knew he didn’t want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. She loved to luxuriate in the presence of this man, and to feel-almost as a sunbather feels the sun-that warm male glow that came out of him to her when they were alone together. She loved him for the way he sat loosely in a chair, for the way he came in a door, or moved slowly across the room with long strides. She loved intent, far look in his eyes when they rested in her, the funny shape of the mouth, and especially the way he remained silent about his tiredness, sitting still with himself until the whiskey had taken some of it away.

He is her entire world.  When he calmly tells Mary that he is leaving her, Mary is devastated.  She's completely blindsided by it, and she is left stunned.  

It didn’t take long, four or five minutes at most, and she say very still through it all, watching him with a kind of dazed horror as he went further and further away from her with each word.

As Mary moves about the house in a daze, Patrick tells her that he will be going out again.  It's the final straw.  Mary can't believe that Patrick would stop loving her.  She can't fathom it, because she doesn't feel anything like that at all.  She feels hurt and devastated, and hitting Patrick over the head with the leg of lamb is likely her way of making Patrick feel some pain too.  Unfortunately (fortunately?) the blow to the head kills Patrick.