The sentence that best describes what Lady Macbeth is praying for in her soliloquy at the beginning of Act 1, Scene 5 is:
a Lady Macbeth wants to be "unsexed," meaning she desires to be more masculine so she can be filled with cruelty to do what is necessary in order to make Macbeth king.
In this soliloquy, she expresses her ambition and willingness to shed any feminine qualities that might make her compassionate or weak, as she believes these traits would hinder her from helping Macbeth fulfill his ambition to take the throne.