This question examines the understanding of terms related to mental states. The correct answer is 'A. Mental focus' because it refers to the concentration of the mind, which can indeed impact various aspects of life. Option B is incorrect because 'evil thoughts' refers to morally bad thoughts, which is not the focus of the question. Option C is incorrect because 'sinful living' refers to a lifestyle involving sin, which is not related to the state of mind affecting happiness. Option D is incorrect because 'steep price' is related to the cost of something and not to mental states.